





Network Working Working Group                                  R. Callon
Request for Comments: 1195                 Digital Equipment Corporation
                                                           December 1990


      Use of OSI IS-IS for Routing in TCP/IP and Dual Environments

Status of this Memo

   This RFC specifies a protocol on the IAB Standards Track for the
   Internet community, and requests discussion and suggestions for
   improvements. Please refer to the current edition of the "IAB
   Official Protocol Standards" for the standardization state and status
   of this protocol. Distribution of this memo is unlimited.

   This RFC is available in both postscript and text versions. Where
   possible, use of the postscript version is recommended. For example,
   this text version may have figures which are less informative or
   missing.

Abstract

   This RFC specifies an integrated routing protocol, based on the OSI
   Intra-Domain IS-IS Routing Protocol, which may be used as an interior
   gateway protocol (IGP) to support TCP/IP as well as OSI. This allows
   a single routing protocol to be used to support pure IP environments,
   pure OSI environments, and dual environments. This specification was
   developed by the IS-IS working group of the Internet Engineering Task
   Force.

   The OSI IS-IS protocol has reached a mature state, and is ready for
   implementation and operational use. The most recent version of the
   OSI IS-IS protocol is contained in ISO DP 10589 [1]. The proposed
   standard for using IS-IS for support of TCP/IP will therefore make
   use of this version (with a minor bug correction, as discussed in
   Annex B).  We expect that future versions of this proposed standard
   will upgrade to the final International Standard version of IS-IS
   when available.

   Comments should be sent to "isis@merit.edu".

Contents

    1   Introduction: Overview of the Protocol
        1.1     What the Integrated IS-IS offers
        1.2     Overview of the ISO IS-IS Protocol
        1.3     Overview of the Integrated IS-IS
        1.4     Support of Mixed Routing Domains



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


        1.5     Advantages of Using Integrated IS-IS

    2   Symbols and Abbreviations

    3   Subnetwork Independent Functions
        3.1     Exchange of Routing Information
        3.2     Hierarchical Abbreviation of IP Reachability Information
        3.3     Addressing Routers in IS-IS Packets
        3.4     External Links
        3.5     Type of Service Routing
        3.6     Multiple LSPs and SNPs
        3.7     IP-Only Operation
        3.8     Encapsulation
        3.9     Authentication
        3.10    Order of Preference of Routes / Dijkstra Computation

    4   Subnetwork Dependent Functions
        4.1     Link Demultiplexing
        4.2     Multiple IP Addresses per Interface
        4.3     LANs, Designated Routers, and Pseudonodes
        4.4     Maintaining Router Adjacencies
        4.5     Forwarding to Incompatible Routers

    5   Structure and Encoding of PDUs
        5.1     Overview of IS-IS PDUs
        5.2     Overview of IP-Specific Information for IS-IS
        5.3     Encoding of IP-Specific Fields in IS-IS PDUs

    6   Security Considerations

    7   Author's Address

    8   References

    A   Inter-Domain Routing Protocol Information
        A.1     Inter-Domain Information Type
        A.2     Encoding

    B   Encoding of Sequence Number Packets
        B.1     Level 1 Complete Sequence Numbers PDU
        B.2     Level 2 Complete Sequence Numbers PDU
        B.3     Level 1 Partial Sequence Numbers PDU
        B.4     Level 2 Partial Sequence Numbers PDU

    C   Dijkstra Calculation and Forwarding
        C.1     SPF Algorithm for IP and Dual Use
        C.2     Forwarding of IP packets




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    D   Use of the Authentication Field
        D.1     Authentication Field in IS-IS packets
        D.2     Authentication Type 1 - Simple Password

    E   Interaction of the Integrated IS-IS with Brouters
        E.1     The Problem
        E.2     Possible Solutions

Figures
        1       ISO Hierarchical Address Structure
        2       An Example
        3       Encoding of Variable Length Fields

1 Introduction: Overview of the Protocol

   The TCP/IP protocol suite has been growing in importance as a multi-
   vendor communications architecture. With the anticipated emergence of
   OSI, we expect coexistence of TCP/IP and OSI to continue for an
   extended period of time. There is a critical need for routers to
   support both IP traffic and OSI traffic in parallel.

   There are two main methods that are available for routing protocols
   to support dual OSI and IP routers. One method, known as "Ships in
   the Night", makes use of completely independent routing protocols for
   each of the two protocol suites. This specification presents an
   alternate approach, which makes use of a single integrated protocol
   for interior routing (i.e., for calculating routes within a routing
   domain) for both protocol suites.

   This integrated protocol design is based on the OSI Intra-domain IS-
   IS routing protocol [1], with IP-specific functions added. This RFC
   is considered a companion to the OSI IS-IS Routing spec, and will
   only describe the required additional features.

   By supporting both IP and OSI traffic, this integrated protocol
   design supports traffic to IP hosts, OSI end systems, and dual end
   systems.  This approach is "integrated" in the sense that the IS-IS
   protocol can be used to support pure-IP environments, pure-OSI
   environments, and dual environments. In addition, this approach
   allows interconnection of dual (IP and OSI) routing domains with
   other dual domains, with IP-only domains, and with OSI-only domains.

   The protocol specified here is based on the work of the IETF IS-IS
   working group.

1.1 What the Integrated IS-IS offers

   The integrated IS-IS provides a single routing protocol which will



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   simultaneously provide an efficient routing protocol for TCP/IP, and
   for OSI. This design makes use of the OSI IS-IS routing protocol,
   augmented with IP-specific information. This design provides explicit
   support for IP subnetting, variable subnet masks, TOS-based routing,
   and external routing. There is provision for authentication
   information, including the use of passwords or other mechanisms. The
   precise form of authentication mechanisms (other than passwords) is
   outside of the scope of this document.

   Both OSI and IP packets are forwarded "as is" -- i.e., they are
   transmitted directly over the underlying link layer services without
   the need for mutual encapsulation. The integrated IS-IS is a dynamic
   routing protocol, based on the SPF (Dijkstra) routing algorithm.

   The protocol described in this specification allows for mixing of
   IP-only, OSI-only, and dual (IP and OSI) routers, as defined below.

   An IP-only IS-IS router (or "IP-only" router) is defined to be a
   router which: (i) Uses IS-IS as the routing protocol for IP, as
   specified in this report; and (ii) Does not otherwise support OSI
   protocols. For example, such routers would not be able to forward OSI
   CLNP packets.

   An OSI-only router is defined to be a router which uses IS-IS as the
   routing protocol for OSI, as specified in [1]. Generally, OSI-only
   routers may be expected to conform to OSI standards, and may be
   implemented independent of this specification.

   A dual IS-IS router (or "dual" router) is defined to be a router
   which uses IS-IS as a single integrated routing protocol for both IP
   and OSI, as specified in this report.

   This approach does not change the way that IP packets are handled.
   IP-only and dual routers are required to conform to the requirements
   of Internet Gateways [4]. The integrated IS-IS protocol described in
   this report outlines an Interior Gateway Protocol (IGP) which will
   provide routing within a TCP/IP routing domain (i.e., autonomous
   system). Other aspects of router functionality (e.g., operation of
   ICMP, ARP, EGP, etc.) are not affected by this proposal.

   Similarly, this approach does not change the way that OSI packets are
   handled. There will be no change at all to the contents nor to the
   handling of ISO 8473 Data packets and Error Reports, nor to ISO 9542
   Redirects and ES Hellos. ISO 9542 IS Hellos transmitted on LANs are
   similarly unchanged. ISO 9542 IS Hellos transmitted on point-to-point
   links are unchanged except for the addition of IP-related
   information.  Similarly, other OSI packets (specifically those
   involved in the IS-IS intra-domain routing protocol) remain unchanged



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   except for the addition of IP-related information.

   This approach makes use of the existing IS-IS packets, with IP-
   specific fields added. Specifically: (i) authentication information
   may be added to all IS-IS packets; (ii) the protocols supported by
   each router, as well as each router's IP addresses, are specified in
   ISO 9542 IS Hello, IS-IS Hello and Link State Packets; (iii)
   internally reachable IP addresses are specified in all Link State
   Packets; and (iv) externally reachable IP addresses, and external
   routing protocol information, may be specified in level 2 Link State
   Packets. The detailed encoding and interpretation of this in
   formation is specified in sections 3, 4, and 5 of this RFC.

   The protocol described in this report may be used to provide routing
   in an IP-only routing domain, in which all routers are IP-only.
   Similarly, this protocol may be used to provide routing in a pure
   dual domain, in which all routers are dual. Finally, this protocol
   may be used to provide routing in a mixed domain, in which some
   routers are IP-only, some routers are OSI-only, and some routers are
   dual. The specific topological restrictions which apply in this
   latter case are described in detail in section 1.4 ("Support of Mixed
   Routing Domains").  The use of IS-IS for support of pure OSI domains
   is specified in [1].

   This protocol specification does not constrain which network
   management protocol(s) may be used to manage IS-IS-based routers.
   Management information bases (MIBs) for managing IP-only, OSI-only,
   and dual routers, compatible with CMIP, CMOT, and/or SNMP, are the
   subject of a separate, companion document [8].

1.2 Overview of the ISO IS-IS Protocol

   The IS-IS Routing Protocol has been developed in ISO to provide
   routing for pure OSI environments. In particular, IS-IS is designed
   to work in conjunction with ISO 8473 (The ISO Connectionless Network
   Layer Protocol [2]), and ISO 9542 (The ISO End System to Intermediate
   System Protocol [3]). This section briefly describes the manner in
   which IS-IS is used to support pure OSI environments. Enhancements
   for support of IP and dual environments are specified elsewhere in
   this report.

   In IS-IS, the network is partitioned into "routing domains". The
   boundaries of routing domains are defined by network management, by
   setting some links to be "exterior links". If a link is marked as
   "exterior", no IS-IS routing messages are sent on that link.

   Currently, ISO does not have a standard for inter-domain routing
   (i.e., for routing between separate autonomous routing domains).



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   Instead, manual configuration is used. The link is statically
   configured with the set of address prefixes reachable via that link,
   and with the method by which they can be reached (such as the DTE
   address to be dialed to reach that address, or the fact that the DTE
   address should be extracted from the IDP portion of the ISO address).

   OSI IS-IS routing makes use of two-level hierarchical routing. A
   routing domain is partitioned into areas. Level 1 routers know the
   topology in their area, including all routers and end systems in
   their area. However, level 1 routers do not know the identity of
   routers or destinations outside of their area. Level 1 routers
   forward all traffic for destinations outside of their area to a level
   2 router in their area. Similarly, level 2 routers know the level 2
   topology, and know which addresses are reachable via each level 2
   router. However, level 2 routers do not need to know the topology
   within any level 1 area, except to the extent that a level 2 router
   may also be a level 1 router within a single area. Only level 2
   routers can exchange data packets or routing information directly
   with external routers located outside of the routing domains.

    +----------------------+-------------------------------+
    |        IDP           |              DSP              |
    +----------------------+-------------------------------+
    .                      .                               .
    .                      .                               .
    .                      .                               .
    +-----+----------------+----------+--------------+-----+
    | AFI |      IDI       |  HO-DSP  |      ID      | SEL |
    +-----+----------------+----------+--------------+-----+

         Figure 1 - ISO Hierarchical Address Structure


   As illustrated in figure 1, ISO addresses are subdivided into the
   Initial Domain Part (IDP), and the Domain Specific Part (DSP). The
   IDP is the part which is standardized by ISO, and specifies the
   format and authority responsible for assigning the rest of the
   address. The DSP is assigned by whatever addressing authority is
   specified by the IDP. The DSP is further subdivided into a "High
   Order Part of DSP" (HO-DSP), a system identifier (ID), and an NSAP
   selector (SEL). The HO-DSP may use any format desired by the
   authority which is identified by the IDP. Together, the combination
   of [IDP, HO-DSP] identify both the routing domain and the area within
   the routing domain. The combination of [IDP,HO-DSP] may therefore be
   referred to as the "Area Address".

   Usually, all nodes in an area have the same area address. However,
   sometimes an area might have multiple addresses. Motivations for



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   allowing this are:

   - It might be desirable to change the address of an area. The most
     graceful way of changing an area from having address A to having
     address B is to first allow it to have both addresses A and B, and
     then after all nodes in the area have been modified to recognize
     both addresses, then one by one the nodes can be modified to
     "forget" address A.

   - It might be desirable to merge areas A and B into one area. The
     method for accomplishing this is to, one by one, add knowledge of
     address B into the A partition, and similarly add knowledge of
     address A into the B  partition.

   - It might be desirable to partition an area C into two areas, A
     and B (where "A" might equal "C", in which case this example
     becomes one of removing a portion of an area). This would be
     accomplished by first introducing knowledge of address A into
     the appropriate nodes (those destined to become area A), and
     knowledge of address B into the appropriate nodes, and then one
     by one removing knowledge of address C.

   Since OSI addressing explicitly identifies the area, it is very easy
   for level 1 routers to identify packets going to destinations outside
   of their area, which need to be forwarded to level 2 routers.

   In IS-IS, there are two types of routers:

   - Level 1 intermediate systems -- these nodes route based on the ID
     portion of the ISO address. They route within an area. They
     recognize, based on the destination address in a packet, whether
     the destination is within the area. If so, they route towards
     the destination. If not, they route to the nearest level 2 router.

   - Level 2 intermediate systems -- these nodes route based on the area
     address (i.e., on the combination of [IDP, HO-DSP]). They route
     towards areas, without regard to the internal structure of an area.
     A level 2 IS may also be a level 1 IS in one area.

   A level 1 router will have the area portion of its address manually
   configured. It will refuse to become a neighbor with a node whose
   area addresses do not overlap its area addresses. However, if level 1
   router has area addresses A,  B, and C, and a neighbor has area
   addresses B and D, then the level 1 router will accept the other node
   as a neighbor.

   A level 2 router will accept another level 2 router as a neighbor,
   regardless of area address. However, if the area addresses do not



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   overlap, the link would be considered by both routers to be "level 2
   only", and only level 2 LSPs would flow on the link. External links
   (to other routing domains) must be from level 2 routers.

   IS-IS provides an optional partition repair function. In the unlikely
   case that a level 1 area become partitioned, this function, if
   implemented, allows the partition to be repaired via use of level 2
   routes.

   IS-IS requires that the set of level 2 routers be connected. Should
   the level 2 backbone become partitioned, there is no provision for
   use of level 1 links to repair a level 2 partition.

   In unusual cases, a single level 2 router may lose connectivity to
   the level 2 backbone. In this case the level 2 router will indicate
   in its level 1 LSPs that it is not "attached", thereby allowing level
   1 routers in the area to route traffic for outside of the domain to a
   different level 2 router. Level 1 routers therefore route traffic to
   destinations outside of their area only to level 2 routers which
   indicate in their level 1 LSPs that they are "attached".

   An end system may autoconfigure the area portion of its address by
   extracting the area portion of a neighboring router's address. If
   this is the case, then an endnode will always accept a router as a
   neighbor. Since the standard does not specify that the end system
   MUST autoconfigure its area address, an end system may be configured
   with an area address. In this case the end system would ignore router
   neighbors with non-matching area addresses.

   Special treatment is necessary for broadcast subnetworks, such as
   LANs. This solves two sets of issues: (i) In the absence of special
   treatment, each router on the subnetwork would announce a link to
   every other router on the subnetwork, resulting in n-squared links
   reported; (ii) Again, in the absence of special treatment, each
   router on the LAN would report the same identical list of end systems
   on the LAN, resulting in substantial duplication.

   These problems are avoided by use of a "pseudonode", which represents
   the LAN. Each router on the LAN reports that it has a link to the
   pseudonode (rather than reporting a link to every other router on the
   LAN). One of the routers on the LAN is elected "designated router".
   The designated router then sends out an LSP on behalf of the
   pseudonode, reporting links to all of the routers on the LAN. This
   reduces the potential n-squared links to n links. In addition, only
   the pseudonode LSP includes the list of end systems on the LAN,
   thereby eliminating the potential duplication (for further
   information on designated routers and pseudonodes, see [1]).




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   The IS-IS provides for optional Quality of Service (QOS) routing,
   based on throughput (the default metric), delay, expense, or residual
   error probability. This is described in greater detail in section
   3.5, and in [1].

1.3 Overview of the Integrated IS-IS

   The integrated IS-IS allows a single routing protocol to be used to
   route both IP and OSI packets. This implies that the same two-level
   hierarchy will be used for both IP and OSI routing. Each area will be
   specified to be either IP-only (only IP traffic can be routed in that
   particular area), OSI-only (only OSI traffic can be routed in that
   area), or dual (both IP and OSI traffic can be routed in the area).

   This proposal does not allow for partial overlap of OSI and IP areas.
   For example, if one area is OSI-only, and an other area is IP-only,
   then it is not permissible to have some routers be in both areas.
   Similarly, a single backbone is used for the routing domain. There is
   no provision for independent OSI and IP backbones.

   Similarly, within an IP-only or dual area, the amount of knowledge
   maintained by routers about specific IP destinations will be as
   similar as possible as for OSI. For example, IP-capable level 1
   routers will maintain the topology within the area, and will be able
   to route directly to IP destinations within the area. However, IP-
   capable level 1 routers will not maintain information about
   destinations outside of the area. Just as in normal OSI routing,
   traffic to destinations outside of the area will be forwarded to the
   nearest level 2 router. Since IP routes to subnets, rather than to
   specific end systems, IP routers will not need to keep nor distribute
   lists of IP host identifiers (note that routes to hosts can be
   announced by using a subnet mask of all ones).

   The IP address structure allows networks to be partitioned into
   subnets, and allows subnets to be recursively subdivided into smaller
   subnets. However, it is undesireable to require any specific
   relationship between IP subnet addresses and IS-IS areas. For
   example, in many cases, the dual routers may be installed into
   existing environments, which already have assigned IP and/or OSI
   addresses. In addition, even if IP addresses are not already pre-
   assigned, the address limitations of IP constrain what addresses may
   be assigned. We therefore will not require any specific relationship
   between IP addresses and the area structure. The IP addresses can be
   assigned completely independently of the OSI addresses and IS-IS area
   structure. As will be described in section 3.2 ("Hierarchical
   Abbreviation of IP Reachability Information"), greater efficiency and
   scaling of the routing algorithm can be achieved if there is some
   correspondence between the IP address assignment structure and the



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   area structure.

   Within an area, level 1 routers exchange link state packets which
   identify the IP addresses reachable by each router. Specifically,
   zero or more [IP address, subnet mask, metric] combinations may be
   included in each Link State Packet. Each level 1 router is manually
   configured with the [IP address, subnet mask, metric] combinations
   which are reachable on each interface. A level 1 router routes as
   follows:

   - If a specified destination address matches an [IP address, subnet
     mask, metric] reachable within the area, the packet is routed via
     level 1 routing.

   - If a specified destination address does not match any [IP address,
     subnet mask, metric] combination listed as reachable within the
     area, the packet is routed towards the nearest level 2 router.

   Flexible use of the limited IP address space is important in order to
   cope with the anticipated growth of IP environments. Thus an area
   (and by implication a routing domain) may simultaneously make use of
   a variety of different address masks for different subnets in the
   area (or domain). Generally, if a specified destination address
   matches more than one [IP address, subnet mask] pair, the more
   specific address is the one routed towards (the one with more "1"
   bits in the mask -- this is known as "best match" routing).

   Level 2 routers include in their level 2 LSPs a complete list of [IP
   address, subnet mask, metric] specifying all IP addresses reachable
   in their area. As described in section 3, this information may be
   obtained from a combination of the level 1 LSPs (obtained from level
   1 routers in the same area), and/or by manual configuration. In
   addition, Level 2 routers may report external reachability
   information, corresponding to addresses which can be reached via
   routers in other routing domains (autonomous systems)

   Default routes may be announced by use of a subnet mask containing
   all zeroes. Default routes should be used with great care, since they
   can result in "black holes". Default routes are permitted only at
   level 2 as external routes (i.e., included in the "IP External
   Reachability Information" field, as explained in sections 3 and 5).
   Default routes are not permitted at level 1.

   The integrated IS-IS provides optional Type of Service (TOS) routing,
   through use of the QOS feature from IS-IS.






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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


1.4 Support of Mixed Routing Domains

   The integrated IS-IS proposal specifically allows for three types of
   routing domains:

   - Pure IP

   - Pure OSI

   - Dual

   In a pure IP routing domain, all routers must be IP-capable. IP-only
   routers may be freely mixed with dual routers. Some fields
   specifically related to OSI operation may be included by dual
   routers, and will be ignored by IP-only routers. Only IP traffic will
   be routed in an pure IP domain. Any OSI traffic may be discarded
   (except for the IS-IS packets necessary for operation of the routing
   protocol).

   In a pure OSI routing domain, all routers must be OSI-capable.  OSI-
   only routers may be freely mixed with dual routers. Some fields
   specifically related to IP operation may be included by dual routers,
   and will be ignored by OSI-only routers. Only OSI traffic will be
   routed in a pure OSI domain. Any IP traffic may be discarded.

   In a dual routing domain, IP-only, OSI-only, and dual routers may be
   mixed on a per-area basis. Specifically, each area may itself be
   defined to be pure IP, pure OSI, or dual.

   In a pure IP area within a dual domain, IP-only and dual routers may
   be freely mixed. Only IP traffic can be routed by level 1 routing
   within a pure-IP area.

   In a pure-OSI area within a dual domain, OSI-only and dual routers
   may be freely mixed. Only OSI traffic can be routed by level 1
   routing within a pure OSI area.

   In a dual area within a dual routing domain only dual routers may be
   used. Both IP and OSI traffic can be routed within a dual area.

   Within a dual domain, if both IP and OSI traffic are to be routed
   between areas then all level 2 routers must be dual.

1.5 Advantages of Using Integrated IS-IS

   Use of the integrated IS-IS protocol, as a single protocol for
   routing both IP and OSI packets in a dual environment, has
   significant advantages over using separate protocols for



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   independently routing IP and OSI traffic.

   An alternative approach is known as "Ships In the Night" (S.I.N.).
   With the S.I.N. approach, completely separate routing protocols are
   used for IP and for OSI. For example, OSPF [5] may be used for
   routing IP traffic, and IS-IS [1] may be used for routing OSI
   traffic. With S.I.N., the two routing protocols operate more or less
   independently. However, dual routers will need to implement both
   routing protocols, and therefore there will be some degree of
   competition for resources.

   Note that S.I.N. and the integrated IS-IS approach are not really
   completely separate options. In particular, if the integrated IS-IS
   is used within a routing domain for routing of IP and OSI traffic, it
   is still possible to use other independent routing protocols for
   routing other protocol suites.

   In the future, optional extensions to IS-IS may be defined for
   routing other common protocol suites. However, such future options
   are outside of the scope of this document. This section will compare
   integrated IS-IS and S.I.N. for routing of IP and OSI only.

   A primary advantage of the integrated IS-IS relates to the network
   management effort required. Since the integrated IS-IS provides a
   single routing protocol, within a single coordinated routing domain
   using a single backbone, this implies that there is less information
   to configure. This combined with a single coordinated MIB simplifies
   network management.

   Note that the operation of two routing protocols with the S.I.N.
   approach are not really independent, since they must share common
   resources. However, with the integrated IS-IS, the interactions are
   explicit, whereas with S.I.N., the interactions are implicit. Since
   the interactions are explicit, again it may be easier to manage and
   debug dual routers.

   Another advantage of the integrated IS-IS is that, since it requires
   only one routing protocol, it uses fewer resources. In particular,
   less implementation resources are needed (since only one protocol
   needs to be implemented), less CPU and memory resources are used in
   the router (since only one protocol needs to be run), and less
   network resources are used (since only one set of routing packets
   need to be transmitted). Primarily this translates into a financial
   savings, since each of these three types of resources cost money.
   This implies that dual routers based on the integrated IS-IS should
   be less expensive to purchase and operate than dual routers based on
   S.I.N.




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


   Note that the operation of two routing protocols with the S.I.N.
   approach are not really independent, since they must share common
   resources. For example, if one routing protocol becomes unstable and
   starts to use excessive resources, the other protocol is likely to
   suffer. A bug in one protocol could crash the other. However, with
   the integrated IS-IS, the interactions are explicit and are defined
   into the protocol and software interactions. With S.I.N., the
   interactions are implicit.

   The use of a single integrated routing protocol similarly reduces the
   likely frequency of software upgrades. Specifically, if you have two
   different routing protocols in your router, then you have to upgrade
   the software any time EITHER of the protocols change. If you make use
   of a single integrated routing protocol, then software changes are
   still likely to be needed, but less frequently.

   Finally, routing protocols have significant real time requirements.
   In IS-IS, these real time requirements have been explicitly
   specified. In other routing protocols, these requirements are
   implicit. However, in all routing protocols, there are real time
   guarantees which must be met in order to ensure correct operation. In
   general, it is difficult enough to ensure compliance with real time
   requirements in the implementation of a single real time system. With
   S.I.N., implementation of two semi-independent real-time protocols in
   a single device makes this more difficult.

   Note that both integrated IS-IS and S.I.N. allow for independence of
   external routes (for traffic from/to outside of the routing domain),
   and allow for independent assignment of OSI and TCP/IP addresses.

2 Symbols and Abbreviations

AA              Administrative Authority
                (a three octet field in the GOSIP version 2.0 NSAP
                address format)

AFI             Authority and Format Identifier
                (the first octet of all OSI NSAP addresses -- identifies
                format of the rest of the address)

CLNP            Connection-Less Network Protocol
                (ISO 8473, the OSI connectionless network layer protocol
                -- very similar to IP)

DFI             DSP Format Identifier
                (a one octet field in the GOSIP version 2.0 NSAP address
                format)




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ES              End System
                (The OSI term for a host)

ES-IS           End System to Intermediate System Routeing Exchange
                Protocol (ISO 9542 -- OSI protocol between routers
                and end systems)

ICD             International Code Designator
                (ISO standard for identifying organizations)

IP              Internetwork Protocol
                (an Internet Standard Network Layer Protocol)

IS              Intermediate System
                (The OSI term for a router)

IS-IS           Intermediate System to Intermediate System Routeing
                Exchange Protocol
                (the ISO protocol for routing within a single
                routing domain)

IS-IS Hello     An Hello packet defined by the IS-IS protocol
                (a type of packet used by the IS-IS protocol)

ISH             An Hello packet defined by ISO 9542 (ES-IS protocol).
                (not the same as IS-IS Hello)

ISO             International Organization for Standardization
                (an international body which is authorized to write
                standards of many kinds)

LSP             Link State Packet
                (a type of packet used by the IS-IS protocol)

NLPID           Network Layer Protocol ID
                (A one-octet field identifying a network layer protocol)

NSAP            Network Service Access Point
                (a conceptual interface point at which the network
                service is made available)

SEL             NSAP Selector
                (the last octet of NSAP addresses, also called NSEL)

OSI             Open Systems Interconnection
                (an international standard protocol architecture)





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RD              Routing Domain
                (the set of routers and end systems using a single
                instance of a routing protocol such as IS-IS)

SNPA            Subnetwork Point of Attachment
                (a conceptual interface at which a subnetwork service
                is provided)

TCP             Transmission Control Protocol
                (an Internet Standard Transport Layer Protocol)

TCP/IP          The protocol suite based on TCP, IP, and related
                protocols (the Internet standard protocol
                architecture)

3 Subnetwork Independent Functions

3.1 Exchange of Routing Information

   The exchange of routing information between routers makes use of the
   normal routing packet exchange as defined in the OSI IS-IS routing
   spec, with additional IP-specific information added to the IS-IS
   routing packets.

   The IS-IS protocol provides for the inclusion of variable length
   fields in all IS-IS packets. These fields are encoded using a "Code,
   Length, Value" triplet, where the code and length are encoded in one
   octet each, and the value has the length specified (from 0 to 254
   octets). IS-IS requires that: "Any codes in a received PDU that are
   not recognised are ignored and passed through unchanged". This
   requirement applies to all routers implementing IS-IS, including
   OSI-only, IP-only, and dual routers. This allows IP-specific
   information to be encoded in a manner which OSI-only routers will
   ignore, and also allows OSI-specific information to be encoded in a
   manner which IP-only routers will ignore.

   IP-capable (i.e., all IP-only and dual) routers need to know what
   network layer protocols are supported by other routers in their area.
   This information is made available by inclusion of a "protocols
   supported" field in all IS-IS Hello and Link State Packets. This
   field makes use of the NLPID (Network Layer Protocol Identifier),
   which is a one-octet value assigned by ISO to identify network level
   protocols. NLPID values have been assigned to ISO 8473 and to IP.

   IP-capable routers need to know the IP address of the adjacent
   interface of neighboring routers. This is required for sending ICMP
   redirects (when an IP-capable router sends an ICMP redirect to a
   host, it must include the IP address of the appropriate interface of



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   the correct next-hop router). This information is made available by
   inclusion of the IP interface address in the IS-IS Hello packets.
   Specifically, each IS-IS Hello packet contains the IP address(es) of
   the interface over which the Hello is transmitted. The IS-IS allows
   multiple IP addresses to be assigned to each physical interface.

   In some cases, it will be useful for IP-capable routers to be able to
   determine an IP address(es) of all other routers at their level
   (i.e., for level 1 routers: all other routers in their area; for
   level 2 routers: all other level 2 routers in the routing domain).
   This is useful whenever an IP packet is to be sent to a router, such
   as for encapsulation or for transmission of network management
   packets. This information is made available by inclusion of IP
   address in LSPs. Specifically, each IS-IS LSP includes one or more IP
   addresses of the router which transmits the LSP. An IP-capable router
   is required to include at least one of its IP addresses in its LSPs,
   and may optionally include several or all of its IP addresses. Where
   a single router operates as both a level 1 and a level 2 router, it
   is required to include the same IP address(es) in its level 1 and
   level 2 LSPs.

   IP-capable routers need to know, for any given IP destination
   address, the correct route to that destination. Specifically, level 1
   routers need to know what IP addresses are reachable from each level
   1 router in their area. In addition, level 1 routers need to find
   level 2 routers (for traffic to IP addresses outside of their area).
   Level 2 routers need to know what IP addresses are reachable
   internally (either directly, or via level 1 routing) from other level
   2 routers, and what addresses are reachable externally from other
   level 2 routers. All of this information is made available by
   inclusion of IP reachable address information in the Link State
   Packets.

   Internal (within the routing domain) and external (outside the
   domain) reachability information is announced separately in level 2
   LSPs. Reachable IP addresses include a default metric, and may
   include multiple TOS-specific metrics. In general, for external
   routes, metrics may be of type "internal" (i.e., directly comparable
   with internal metrics) or of type "external" (i.e., not comparable
   with the internal metric). A route using internal metrics (i.e.,
   either announced as "IP internal reachability information", or
   announced as "IP external reachability information" with an internal
   metric) is always preferred to a route using external metrics (i.e.,
   announced as "IP external reachability information", with an external
   metric).

   The detailed encoding of the IP-specific information included in
   routing packets is provided in section 5 (Structure and Encoding of



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   PDUs).

3.2 Hierarchical Abbreviation of IP Reachability Information

   Level 2 routers include in their level 2 LSPs a list of all [IP
   address, subnet mask, metric] combinations reachable in their area.
   In general, this information may be determined from the level 1 LSPs
   from all routers in the area. If we ignore resource constraints, then
   it would be permissible for a level 2 router to simply duplicate all
   [IP address, subnet mask, metric] entries from all level 1 routers in
   its area (with appropriate metric adjustment), for inclusion in its
   level 2 LSP. However, in order for hierarchical routing to scale to
   large routing domain sizes, it is highly desired to abbreviate the
   reachable address information.

   This is accomplished by manual configuration of summary addresses.
   Each level 2 router may be configured with one or more [IP address,
   subnet mask, metric] entries for announcement in their level 2 LSPs.

   The set of reachable addresses obtained from level 1 LSPs is compared
   with the configured reachable addresses. Redundant information
   obtained from level 1 LSPs is not included in level 2 LSPs. Generally
   it is expected that the level 2 configured information will specify
   more inclusive addresses (corresponding to a subnet mask with fewer
   bits set to 1). This will therefore allow one configured
   address/submask pair (or a small number of such pairs) to
   hierarchically supercede the information corresponding to multiple
   entries in level 1 LSPs.

   The manually configured addresses are included in level 2 LSPs only
   if they correspond to at least one address which is reachable in the
   area. For manually configured level 2 addresses, the associated
   metric values to announce in level 2 LSPs are also manually
   configured. The configured addresses will supercede reachable address
   entries from level 1 LSPs based only on the IP address and subnet
   mask -- metric values are not considered when determining if a given
   configured address supercedes an address obtained from a level 1 LSP.

   Any address obtained from a level 1 LSP which is not superceded by
   the manually configured information is included in the level 2 LSPs.
   In this case, the metric value announced in the level 2 LSPs is
   calculated from the sum of the metric value announced in the
   corresponding level 1 LSP, plus the distance from the level 2 router
   to the appropriate level 1 router. Note: If this sum results in a
   metric value greater than 63 (the maximum value that can be reported
   in level 2 LSPs), then the value 63 must be used. Delay, expense, and
   error metrics (i.e., those TOS metrics other than the default metric)
   will be included only if (i) the level 2 router supports the specific



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   TOS; (ii) the path from the level 2 router to the appropropriate
   level 1 router is made up of links which support the specific TOS;
   and (iii) the level 1 router which can reach the address directly
   also supports the specific TOS for this route, as indicated in its
   level 1 LSP.

   In general, the same [IP address, subnet mask] pair may be announced
   in level 1 LSPs sent by multiple level 1 routers in the same area. In
   this case (assuming the entry is not superceded by a manually
   configured entry), then only one such entry shall be included in the
   level 2 LSP. The metric value(s) announced in level 2 LSPs correspond
   to the minimum of the metric value(s) that would be calculated for
   each of the level 1 LSP entries.

   A level 2 router will have IP addresses which are directly reachable
   via its own interfaces. For purposes of inclusion of IP reachable
   address information in level 2 LSPs, these "directly reachable"
   addresses are treated exactly the same as addresses received in level
   1 LSPs.

   Manually configured addresses may hierarchically supercede multiple
   level 1 reachable address entries. However, there may be some IP
   addresses which match the manually configured addresses, but which
   are not reachable via level 1 routing. If a level 2 router receives
   an IP packet whose IP address matches a manually configured address
   which it is including in its level 2 LSP, but which is not reachable
   via level 1 routing in the area, then the packet must be discarded.
   In this case, an error report may be returned (as specified in RFC
   1009), with the reason for discard specifying destination
   unreachable.






           Figure 2 - An Example Routing Domain (not shown)

   An example is illustrated in figure 2. Suppose that the network
   number for the entire routing domain is 17 (a class A network).
   Suppose each area is assigned a subnet number consisting of the next
   8 bits. The area may be further subdivided by assigning the next
   eight bits to each LAN in the area, giving each a 24 bit subnet mask
   (counting the network and subnet fields). Finally 8 bits are left for
   the host field. Suppose that for a particular area (given subnet
   number 17.133) there are a number of IP capable level 1 routers
   announcing (in the special IP entry in their level 1 LSPs) subnets
   17.133.5, 17.133.43, and 17.133.57.



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   Suppose that in this example, in order to save space in level 2 LSPs,
   the level 2 routers in this area are configured to announce subnet
   17.133. Only this one address needs to be announced in level 2 LSPs.
   Thus if an IP packet comes along for an address in subnet 17.133.5,
   17.133.43 or 17.133.57, then other level 2 routers, in other areas,
   will know to pass the traffic to this area.

   The inclusion of 17.133 in level 2 LSPs means that the three subnet
   addresses starting with 17.133 do not all have to be listed
   separately in level 2 LSPs.

   If any traffic comes along that is for an unreachable address such as
   17.133.124.7, then level 2 routers in other areas in this particular
   domain will think that this area can handle this traffic, will
   forward traffic to level 2 routers in this area, which will have to
   discard this traffic.

   Suppose that subnet number 17.133.125 was actually reachable via some
   other area, such as the lower right hand area. In this case, the
   level 2 router in the left area would be announcing (in its level 2
   LSPs according to manually configured information) reachability to
   subnet 17.133. However, the level 2 router in the lower right area
   would be announcing (in its level 2 LSPs according to information
   taken from its received level 1 LSPs), reachability to subnet
   17.133.125. Due to the use of best match routing, this works
   correctly. All traffic from other areas destined to subnet 17.133.125
   would be sent to the level 2 router in the lower right area, and all
   other traffic to subnet 17.133 (i.e., traffic to any IP address
   starting with 17.133, but not starting with 17.133.125) would be sent
   to the level 2 router in the leftmost area.

3.3 Addressing Routers in IS-IS Packets

   The IS-IS packet formats explicitly require that OSI-style addresses
   of routers appear in the IS-IS packets. For example, these addresses
   are used to determine area membership of routers. It is therefore
   necessary for all routers making use of the IS-IS protocol to have
   OSI style addresses assigned. For IP-only routers, these addresses
   will be used only in the operation of the IS-IS protocol, and are not
   used for any other purpose (such as the operation of EGP, ICMP, or
   other TCP/IP protocols).

   For OSI-only and dual routers, assignment of NSAP addresses is
   straight forward, but is outside of the scope of this specification.
   Address assignment mechanisms are being set up by standards bodies
   which allow globally unique OSI NSAP addresses to be assigned. All
   OSI-only and dual routers may therefore make use of normal OSI
   addresses in the operation of the IS-IS protocol.



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   For IP-only routers, there are two ways in which NSAP addresses may
   be obtained for use with the IS-IS protocol.

   1) For those environments in which OSI is being used, or in which it
      is anticipated that OSI will be used in the future, it is
      permissible to obtain NSAP address assignments in the normal
      manner, assign normal NSAP addresses to IP-only routers, and use
      these addresses in the operation of IS-IS. This approach is
      recommended even for pure IP routing domains, as it will simplify
      future migration from IP-only to dual operation.

   2) In some cases, routers may have only TCP/IP addresses, and it may
      be undesireable to have to go through the normal mechanisms for
      assignment of NSAP addresses. Instead, an alternate mechanim is
      provided below for algorithmically generating a valid OSI style
      address from existing IP address and autonomous system number
      assignments.

   Where desired, for IP-only routers, for use in IS-IS packet formats
   only, OSI-style addresses (compatible with the USA GOSIP version 2.0
   NSAP address format [9]) may be derived as follows:

        AFI       1 octet       value "47" (specifies ICD format)

        ICD       2 octet       value "00 05" (specifies Internet/Gosip)

        DFI       1 octet       value "xx"

        AA        3 octets      value "xx xx xx" (specifies special
                                IP-only use of NSAPs)

        Reserved  2 octets      must be "00 00"

        RD        2 octets      contains autonomous system number

        Area      2 octets      must be assigned as described below

        ID        6 octets      must be assigned as described below

        SEL       1 octet       used as described below

   The AFI value of "47" and the ICD value of "00 05" specifies the
   Gosip Version 2.0 addressing format. The DFI number of "xx" and the
   AA of "xx xx xx" specify that this special NSAP address format is
   being used, solely for IS-IS packet formats in an IP-only
   environment. The reserved field must contain "00 00", as specified in
   GOSIP version 2.0.




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   The routing domain field contains the Autonomous System number.
   Strictly speaking, this is not necessary, since the IS-IS packets are
   exchanged within a single AS only. However, inclusion of the AS
   number in this address format will ensure correct operation in the
   event that routers from separate routing domains/ASs are incorrectly
   placed on the same link. The AS number in this context is used only
   for definition of unique NSAP addresses, and does not imply any
   coupling with exterior routing protocols.

   The Area field must be assigned by the authority responsible for the
   routing domain, such that each area in the routing domain must have a
   unique Area value.

   The ID must be assigned by the authority responsible for the routing
   domain. The ID must be assigned such that every router in the routing
   domain has a unique value. It is recommended that one of the
   following methods is used:

   1)use a unique IEEE 802 48 bit station ID

   2)use the value hex "02 00" prepended to an IP address of the router.

   IEEE 802 addresses, if used, must appear in IEEE canonical format.

   Since the IEEE 802 station IDs are assigned to be globally unique,
   use of these values clearly assures uniqueness in the area. Also, all
   assigned IEEE 802 station IDs have the global/local bit set to zero.
   Prepending the indicated pattern to the front of the IP address
   therefore assures that format (2) illustrated above cannot produce
   addresses which collide with format (1). Finally, to the extent that
   IP addresses are also globally unique, format (2) will produce unique
   IDs for routers.

   The indicated hex value is specified in IEEE 802 canonical form [10].
   In IEEE 802 addresses, the multicast bit is the least significant bit
   of the first byte. The global/local bit is the next least significant
   bit of the first byte. The indicated prefix therefore sets the
   global/local bit to 1, and all other bits in the first two octets to
   0.

   Note that within an area, whether ISO addresses are configured into
   the routers through ISO address assignment, or whether the ISO-style
   address is generated directly from the AS number and IP address, all
   routers within an area must have the same high order part of address
   (AFI, ICD, DFI, AA, RD, and Area). This ISO-style address is used in
   IS-IS Hello messages and is the basis by which routers recognize
   whether neighbor nodes are in or out of their area.




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3.4 External Links

   External connectivity (i.e., communications with routers outside of
   the routing domain) is done only by level 2 routers. The ISO version
   of IS-IS allows external OSI routes to be reported as "reachable
   address prefixes" in level 2 LSPs. The integrated IS-IS also allows
   external IP reachable addresses (i.e., IP addresses reachable via
   inter-domain routing) to be reported in level 2 LSPs in the "IP
   external reachability information" field. External OSI and external
   IP routes are handled independently.

   The routes announced in IP external reachability information entries
   include all routes to outside of the routing domain. This includes
   routes learned from OSPF, EGP, RIP, or any other external protocol.

   External routes may make use of "internal" or "external" metrics.
   Internal metrics are comparable with the metrics used for internal
   routes. Thus in choosing between an internal route, and an external
   route using internal metrics, the metric values may be directly
   compared. In contrast, external metrics cannot be directly compared
   with internal metrics. Any route defined solely using internal
   metrics is always preferred to any route defined using external
   metrics. When an external route using external metrics must be used,
   the lowest value of the external metric is preferred regardless of
   the internal cost to reach the appropriate exit point.

   It is useful, in the operation of external routing protocols, to
   provide a mechanism for border routers (i.e., routers in the same
   routing domain, which have the ability to route externally to other
   domains) to determine each other's existence, and to exchange
   external information (in a form understood only by the border routers
   themselves). This is made possible by inclusion of "inter-domain
   routing protocol information" fields in level 2 LSPs. The inter-
   domain routing protocol information field is not included in
   pseudonode LSPs.

   In general there may be multiple types of external inter-domain
   routing protocol information exchanged between border routers. The
   IS-IS therefore specifies that each occurance of the inter-domain
   routing protocol information field include a "type" field, which
   indicates the type of inter-domain routing protocol information
   enclosed. Values to be used in the type field will be specified in
   future versions of the "Assigned Numbers" RFC. Initial values for
   this field are specified in Annex A of this specification.

   Information contained in the inter-domain routing protocol
   information field will be carried in level 2 LSPs, and will therefore
   need to be stored by all level 2 routers in the domain. However, only



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   those level 2 routers which are directly involved in external routing
   will use this information. In designing the use of this field, it is
   important to carefully consider the implications that this may have
   on storage requirements in level 2 routers (including those level 2
   routers which are not directly involved in external routing).

   The protocols used to exchange routing information directly between
   border routers, and external routers (in other routing domains /
   autonomous systems) are outside of the scope of this specification.

3.5 Type of Service Routing

   The integrated IS-IS protocol provides IP Type of Service (TOS)
   routing, through use of the Quality of Service (QOS) feature of IS-
   IS. This allows for routing on the basis of throughput (the default
   metric), delay, expense, or residual error probability. Note than any
   particular packet may be routed on the basis of any one of these four
   metrics. Routing on the basis of general combinations of metrics is
   not supported.

   The support for TOS/QOS is optional. If a particular packet calls for
   a specific TOS, and the correct path from the source to destination
   is made up of routers all of which support that particular TOS, then
   the packet will be routed on the optimal path. However, if there is
   no path from the source to destination made up of routers which
   support that particular type of service, then the packet will be
   forwarded using the default metric instead. This allows for TOS
   service in those environments where it is needed, while still
   providing acceptable service in the case where an unsupported TOS is
   requested.

   NOTE - IP does not have a cost TOS. There is therefore no mapping of
   IP TOS metrics which corresponds to the minimum cost metric.

   The IP TOS field is mapped onto the four available metrics as
   follows:

   Bits 0-2 (Precedence):  This field does not affect the route, but
                           rather may affect other aspects of packet
                           forwarding.

   Bits 3 (Delay), 4 (Throughput) and 5 (Reliability):

           000     (all normal)            Use default metric

           100     (low delay)             Use delay metric

           010     (high throughput)       Use default metric



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           001     (high reliabiity)       Use reliability metric

           other                           Use default metric

3.6 Multiple LSPs and SNPs

   In some cases, IS-IS packets (specifically Link State Packets and
   Complete Sequence Number Packets) may be too large to fit into one
   packet. The OSI IS-IS [1] allows for LSPs and CSNPs to be split into
   multiple packets. This is independent of ISO 8473 segmentation, and
   is also independent of IP fragmentation. Use of independent multiple
   packets has the advantages (with respect to segmentation or
   fragmentation) that: (i) when information in the IS-IS changes, only
   those packets effected need to be re-issued; (ii) when a single
   packet is received, it can be processed without the need to receive
   all other packets of the same type from the same router before
   beginning processing.

   The Integrated IS-IS makes use of the same multiple packet function,
   as defined in [1]. IP-specific fields in IS-IS packets may be split
   across multiple packets. As specified in section 5 ("Structure and
   Encoding of PDUs"), some of the IP-specific fields (those which may
   be fairly long) may be split into several occurences of the same
   field, thereby allowing splitting of the fields across different
   packets.

   Multiple LSPs from the same router are distinguished by LSP number.
   Generally, most variable length fields may occur in an LSP with any
   LSP number. Some specific variable length fields may be required to
   occur in LSP number 0. Except where explicitly stated otherwise, when
   an IS-IS router issues multiple LSPs, the IP-specific fields may
   occur in an LSP with any LSP number.

   Complete Sequence Number Packets may be split into multiple packets,
   with the range to which each packet applies explicitly reported in
   the packet. Partial Sequence Number Packets are inherently partial,
   and so can easily be split into multiple packets if this is
   necessary. Again, where applicable, IP-specific fields may occur in
   any SNP.

3.7 IP-Only Operation

   For IP-only routers, the format for IS-IS packets remains unchanged.
   However, there are some variable length fields from the IS-IS packets
   that can be omitted. Specifically:






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   IS-IS Hello Packets:

           - no change

   IS-IS Link State Packets:

           - the "End Systems Neighbours" entries are omitted

           - the "Prefix Neighbours" entries are omitted

   IS-IS Sequence Number Packets:

           - no change

3.8 Encapsulation

   Future versions of the Integated IS-IS may specify optional
   encapsulation mechanisms for partition repair, and for forwarding
   packets through incompatible routers (i.e., for forwarding OSI
   packets through IP-only routers, and forwarding IP packets through
   OSI-only routers). The details of encapsulation and decapsulation are
   for further study. Routers complying with the Integrated IS-IS are
   not required to implement encapsulation nor decapsulation.

3.9 Authentication

   The authentication field allows each IS-IS packet to contain
   information used to authenticate the originator and/or contents of
   the packet.  The authentication information contained in each packet
   is used to authenticate the entire packet, including OSI and IP
   parts. If a packet is received which contains invalid authentication
   information, then the entire packet is discarded. If an LSP or SNP is
   split into multiple packets (as described in section 3.6), then each
   is authenticated independently.

   Use of the authentication field is optional. Routers are not required
   to be able to interpret authentication information. As with other
   fields in the integrated IS-IS, if a router does not implement
   authentication then it will ignore any authentication field that may
   be present in an IS-IS packet.

   Annex D specifies a proposed use of the authentication field.

3.10 Order of Preference of Routes / Dijkstra Computation

   We define the term "IP reachability entry" to mean the combination of
   the [IP address, subnet mask]. The Dijkstra calculation must
   calculate routes to each distinct IP reachability entry. For the



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   Dijkstra calculation, each IP reachability entry can be treated in
   much the same manner as an OSI end system. Naturally, each IP
   reachability entry is treated as distinct from any OSI end systems
   which may also be reachable in the same area or routing domain.

   For any particular IP reachability entry, this is the same as another
   entry if and only if: (i) the subnet masks are identical; and (ii)
   for each bit in the subnet mask which has the value "1", the IP
   address is identical. This can easily be tested by zeroing those bits
   in the IP address which correspond to a zero bit in the mask, and
   then treating the entry as a 64 bit quantity, and testing for
   equality between different 64 bit quantities. The actual calculation
   of routes to IP reachability entries is therefore no more complex
   than calculation of routes to OSI end systems (except for the
   replacement of a 48-bit test with a 64-bit test).

   The Dijkstra computation does not take into consideration whether a
   router is IP-only, OSI-only, or dual. The topological restrictions
   specified in section 1.4 ensure that IP packets will only be sent via
   IP-capable routers, and OSI packets will only be sent via OSI-capable
   routers.

   The Integrated IS-IS prefers routes within the area (via level 1
   routing) whenever possible. If level 2 routes must be used, then
   routes within the routing domain (specifically, those routes using
   internal metrics) are prefered to routes outside of the routing
   domain (using external metrics).

   The Integrated IS-IS protocol makes use of "best match" routing of IP
   packets. This implies that a particular destination address may match
   more than one entry in the forwarding database. If a particular IP
   packet has a destination address which matches two different IP
   reachability entries, then the entry who's mask contains the most "1"
   bits is preferred.

   IP packets whose destination is a router are routed the same way as
   any other IP packet, by forwarding first to the appropriate subnet,
   and then forwarding on that subnet to the destination host (which
   just happens to be a router in this case). In particular, the IP
   forwarding database does not contain explicit routes to the
   individual "IP interface addresses" listed by each router in its LSP.

   However, host routes (routes with a subnet mask of all ones) may of
   course be included in the IP reachability entries, and will be
   handled in the same manner as other IP reachability entries.

   In order to ensure correct interoperation of different router
   implementations, it is necessary to specify the order of preference



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   of possible routes. For OSI destinations, this is outside of the
   scope of this report. For IP destinations, this is specified in
   section 3.10.1 and 3.10.2 below. Annex C specifies a detailed
   Dijkstra calculation and forwarding algorithm which is compatible
   with the order of preference of routes specified here.

   With IS-IS, if a route to a given destination is advertised, or a
   link between routers is advertised, then metric values associated
   with some or all of the specified TOS metric types may be associated
   with that destination or link. However, the default metric must
   always be available. Normally this ensures that if a route using any
   TOS metric is available, then a route using the default metric will
   also be available. The only exception to this is where the
   corresponding route using the default metric has a total cost (within
   the area, or within the level 2 backbone) greater than MaxPathMetric.

   In determining the route to a particular destination for a specified
   TOS, only routes using either the requested TOS metric, or the
   default TOS metric, are considered.

3.10.1 Order of Preference of Routes In Level 1 Routing

   If a given destination is reachable within an area via a route using
   either the requested TOS or the default TOS, then the IS-IS will
   always make use of a path within the area (via level 1 routing),
   regardless of whether an alternate path exists outside of the area
   (via level 2 routing). In this case, routes within the area are
   selected as follows:

   1) Amongst routes in the area, if the specified destination
      address matches more than one [IP address, subnet mask] pair,
      then the more specific address match (the one with more "1"
      bits in the mask) is prefered.

   2) Amongst routes in the area to equally specific address
      matches, routes on which the requested TOS (if any) is
      supported are always prefered to routes on which the
      requested TOS is not supported.

   3) Amongst routes in the area of the same TOS to equally
      specific address matches, the shortest routes are prefered.
      For determination of the shortest path, if a route on which
      the specified TOS is supported is available, then the
      specified TOS metric is used, otherwise the default metric
      is used. Amongst routes of equal cost, load splitting may
      be performed as specified in [1].

   For a level 1 only router (i.e., a router which does not take part in



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   level 2 routing, or a level 2 router which is not "attached"), if a
   given destination is not reachable within an area, level 1 routing
   will always route to a level 2 router as follows:

   1) Amongst routes in the area to attached level 2 routers,
      routes on which the requested TOS (if any) is supported
      are always prefered to routes on which the requested TOS
      is not supported.

   2) Amongst routes in the area of the same TOS to attached
      level 2 routers, the shortest routes are prefered. For
      determination of the shortest path, if a route on which
      the specified TOS is supported is available, then the
      specified TOS metric is used, otherwise the default
      metric is used. Amongst routes of equal cost,
      loadsplitting may be performed as specified in [1].

3.10.2 Order of Preference of Routes in Level 2 Routing

   For those level 2 routers which also take part in level 1 routing,
   routes learned via level 1 routing, using either the requested TOS or
   the default TOS, are always prefered to routes learned through level
   2 routing. For destinations which are not reachable via level 1
   routing, or for level 2 only routers (routers which do not take part
   in level 1 routing), then level 2 routes are selected as follows:

   1) Routes using internal metrics only are always preferred
      to routes using external metrics.

   2) If a route using internal metrics only is available:

      a) If the specified destination address matches more
         than one [IP address, subnet mask] pair, then the more
         specific address match (i.e., the largest number of
         "1"s present in the subnet mask) is prefered.

      b) Amongst routes with equally specific address matches
         (i.e., an equal number of "1"s present in the subnet
         mask), routes on which the requested TOS (if any) is
         supported are always preferred to routes on which the
         requested TOS is not supported.

      c) Amongst routes of the same TOS with an equally specific
         address matches, the shortest path is prefered. For
         determination of the shortest path, if a route on which
         the specified TOS is supported is available, then the
         specified TOS metric is used, otherwise the default
         metric is used. Amongst routes of equal cost,



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         loadsplitting may be performed as specified in [1].

         NOTE: Internal routes (routes to destinations announced
         in the "IP Internal Reachability Information" field),
         and external routes using internal metrics (routes to
         destinations announced in the "IP External Reachability
         Information" field, with a metric of type "internal")
         are treated identically for the purpose of the order of
         preference of routes, and the Dijkstra calculation.

   3) If a route using internal metrics only is not available,
      but a route using external metrics is available:

      a) If the specified destination address matches more than
         one [IP address, subnet mask] pair, then the more
         specific address match is prefered.

         NOTE: For external routes, the subnet mask will normally
         correspond precisely to the network number. This implies
         that this test will always discover equal length matching
         strings.  However, this test is included to allow future
         migration to more general handling of external addresses.

      b) Amongst routes with equally specific matches, routes on
         which the requested TOS (if any) is supported are always
         preferred to routes on which the requested TOS is not
         supported. NOTE: for external routes, the route is
         considered to support the requested TOS only if the
         internal route to the appropriate border router
         supports the requested TOS, and the external route
         reported by the border router also supports the
         requested TOS.

      c) Amongst routes of the same TOS with an equal length
         matching address string, the shortest path is prefered.
         For determination of the shortest path:

         (i)  Routes with a smaller announced external metric
              are always prefered.

         (ii) Amongst routes with an equal external metric,
              routes with a shorter internal metric are prefered.
              Amongst routes of equal cost, loadsplitting may be
              performed as specified in [1].

   For level 2 routers which are announcing manually configured summary
   addresses in their level 2 LSPs, in some cases there will exist IP
   addresses which match the manually configured addresses, but which do



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   not match any addresses which are actually reachable via level 1
   routing in the area. Generally, packets to such addresses are handled
   according to the following rules:

   1) If the specified destination is reachable via level 1 routing,
      then according to the order of preference of routes specified
      above, the packet will be delivered via level 1 routing.

   2) If the specified destination is not reachable via level 1 routing,
      but is reachable via 2 routing, and there are other level 2
      routers which offer more desireable routes according to the
      rules specified above (for example a route with a more specific
      match, or a route with an equally specific match which supports
      the correct TOS), then level 2 routing will forward the packet
      according to the more desireable route.

   3) If the specified destination is not reachable via level 1 routing,
      and the manually configured summary address advertised by this
      router (the router which has received the packet and is trying
      to forward it) represents the most desireable route, then the
      destination is unreachable and the packet must be discarded.

4 Subnetwork Dependent Functions

4.1 Link Demultiplexing

   Dual routers may receive a combination of OSI packets, and IP
   packets. It is necessary for the dual routers to be able to clearly
   and unambiguously distinguish the two protocol suites.

   This problem is not unique to the integrated IS-IS routing protocol.
   In fact, this problem will occur in any multi-protocol environment.
   This problem is currently being worked on independently, and is
   outside of the scope of this specification.

   In general, the link type is a configuration parameter. For example,
   whether to use PPP, HDLC, or some other point-to-point protocol over
   a point-to-point link would be configured. For any particular link
   type, a method must be defined for encapsulation of both OSI and IP
   packets. Definition of such methods for common link types is outside
   of the scope of this specification.

   IP packets are encapsulated directly over the underlying link layer
   service, using the normal method for transmssion of IP packets over
   each type of link. Similarly OSI packets are encapsulated directly
   over the underlying link layer service, using the normal method for
   transmission of OSI packets over each type of link. Finally, note
   that IS-IS packets are encapsulated using the normal method for



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   transmission of OSI packets over any particular link type. This
   implies that all IS-IS routers, including IP-only routers, must be
   able to receive IS-IS packets using the normal encapsulation for OSI
   packets.

4.2 Multiple IP Addresses per Interface

   The integrated IS-IS allows each router to have multiple IP addresses
   for each physical interface, up to the maximum number which may be
   contained in a single "IP Interface Address" field (i.e., up to a
   maximum of 63 addresses per interface). For example, where there are
   two logical subnets on the same LAN, the interface may have two IP
   addresses, one corresponding to each logical subnet. Each IS-IS Hello
   packet contains a list of IP addresses associated with the physical
   interface over which the Hello is transmitted.

   It is permissible to implement routers which conform to the
   Integrated IS-IS specification which restrict the number of IP
   addresses per interface. However, IP-capable routers must be able to
   interact correctly with other routers which assign multiple IP
   addresses per physical interface (up to the maximum of 63 addresses
   per interface).

   Where appropriate (for example, in some cases on point-to-point
   links), some interfaces may have no IP addresses assigned. In this
   case, the IS-IS Hello transmitted on that interface may omit the IP
   Interface Address field, or may include the IP Interface Address
   field with zero entries.

4.3 LANs, Designated Routers, and Pseudonodes

   The maintenance of designated routers and pseudonodes is specified in
   [1], and is not changed by this proposal. In the case that IP-only
   and dual routers (or OSI-only and dual routers) are mixed on the same
   LAN in a pure IP area (or a pure OSI area, respectively), any router
   on the LAN may be elected designated router.

   However, there is a fundamental difference in the way that OSI and
   TCP/IP deal with LANs, and other broadcast subnetworks.

   With OSI, the use of the ES-IS protocol (ISO 9542) allows the end
   systems and routers to automatically determine their connectivity,
   thereby allowing all end systems on the LAN to potentially route via
   any of the routers on the LAN.

   In contract, TCP/IP explictly assigns subnet identifiers to each
   local area network. In some cases, a single physical LAN could have
   multiple subnet identifiers assigned to it. In this case, end systems



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   (hosts) which have an address on one logical subnet are explicitly
   precluded from sending IP packets directly to a router whose address
   places it on a different logical subnet. Each router is manually
   configured to know which subnets it can reach on each interface. In
   the case that there are multiple logical subnets on the same LAN,
   each router can only exchange IP packets with those end systems which
   are on the same logical subnet. This implies that it is not
   sufficient for the pseudonode LSP to announce all subnets on the LAN
   (i.e., all [IP address, subnet mask] pairs reachable on the LAN).

   It is therefore necessary for each router to announce in its LSPs
   those subnets which it can reach on each interface, including
   interfaces to broadcast subnetworks such as LANs. The pseudonode LSP
   does not specify the IP addresses which are reachable on the LAN
   (i.e., does not contain the the IP reachability field).

   As specified elsewhere (see the forthcoming update to the
   "Requirements of IP Gateways" [4]), routers may send ICMP redirects
   only if: (i) the IP packet is being forwarded over the same physical
   interface over which it arrived; and (ii) the source address of the
   forwarded IP packet, the IP address of this router's interface (as
   indicated by the source address of the ICMP redirect), and the IP
   address of the router to which the packet is being redirected (again,
   as indicated in the ICMP redirect) are all on the same IP subnet.

4.4 Maintaining Router Adjacencies

   The IS-IS determines whether an adjacency is to be established
   between two routers using means which are independent of the IP
   interface addresses of the routers. Where multiple logical subnets
   occur on the same physical LAN, this potentially allows adjacencies
   to be brought up between two routers which share physical
   connectivity to each other, but which don't have a logical subnet in
   common. IP-capable IS-IS routers therefore must be able to forward IP
   packets over existing adjacencies to routers with which they share
   physical connectivity, even when the IP address of the adjacent
   interface of the neighboring router is on a different logical IP
   subnet.

   For point-to-point links, IS-IS requires exchange of ISO 9542 ISHs,
   as the first step in establishing the link between routers. All IS-IS
   routers are therefore required to transmit and receive ISO 9542 ISH
   packets on point-to-point links.

   The "protocols supported" field (defined in section 5 below) must be
   present in all IS-IS Hello packets sent by dual and IP-only routers.
   If this field is missing, then it is assumed that the packet was
   transmitted by an OSI-only router. Similarly, those 9542 ISHs sent



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   over point-to-point links, where there is (or may be) another IS-IS
   router at the other end of the point-to-point link, must also
   contains the "protocols supported" field. Note that if this field is
   mistakenly sent in a 9542 ISH where there is an ordinary OSI-only End
   System at the other end of the link, then (in accordance to ISO 9542)
   the End System is required to ignore the field and interpret the ISH
   correctly. It is therefore safe to always include this field in ISHs
   sent over point-to-point links.

   Dual routers must operate in a dual fashion on every link in the
   routing domain over which they are running IS-IS. Thus, the value of
   the "protocols supported" field must be identical on every link
   (i.e., for any one router running IS-IS, all of the Hellos and LSPs
   transmitted by it must contain the same "protocols supported"
   values).

4.5 Forwarding to Incompatible Routers

   There may be times when a dual router has to forward an IP packet to
   an OSI-only router, or forward an OSI packet to an IP-only router. In
   this case the packet must be discarded. An error report may be
   transmitted, in accordance with the IP or ISO 8473 specification
   (respectively). The reason for discard specified in the error report
   should specify "destination host unreachable" (for IP), or
   "destination unreachable" (for OSI).

   Similarly, due to errors, in some cases an IP-only router may have to
   forward an IP packet to an OSI-only router. Again, the packet must be
   discarded, as specified above. This may only occur if IP-only and
   OSI-only routers occur in the same area, which is a configuration
   error.

5 Structure and Encoding of PDUs

   This clause describes the additional packet fields for use of the ISO
   IS-IS Intra-Domain Routing protocol in pure IP and dual environments.
   Specifically, the same packet types are used as in IS-IS [1], and all
   fixed fields remain the same. Additional variable length fields are
   defined in this section.

5.1 Overview of IS-IS PDUs

   The packets used in IS-IS routing protocol fall into three main
   classes: (i) Hello Packets; (ii) Link State Packets (LSPs); and (iii)
   Sequence Number Packets (SNPs).

   Hello packets are used to initialize and maintain adjacencies between
   neighboring routers. There are three types of IS-IS Hello packets:



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   (i) "Level 1 LAN IS to IS Hello PDUs" are used by level 1 routers on
   broadcast LANs. (ii) "Level 2 LAN IS to IS Hello PDUs" are used by
   level 2 routers on broadcast LANs. (iii) "Point-to-Point IS to IS
   Hello PDUs" are used on non-broadcast media, such as point-to-point
   links, or general topology subnetworks.

   On point-to-point links, the exchange of ISO 9542 ISHs (intermediate
   system Hellos) is used to initialize the link, and to allow each
   router to know if there is a router on the other end of the link,
   before IS-IS Hellos are exchanged. All routers implementing IS-IS
   (whether IP-only, OSI-only, or dual), if they have any interfaces on
   point-to-point links, must therefore be able to transmit ISO 9542
   ISHs on their point-to-point links.

   Link State Packets (LSPs) are used to exchange link state
   information. There are two types of LSPs: (i) "Level 1 Link State
   PDUs" are transmitted by level 1 routers. (ii) "Level 2 Link State
   PDUs" are transmitted by level 2 routers. Note that level 2 routers
   will, in most cases, also be level 1 routers, and will therefore
   transmit both sorts of LSPs.

   Sequence number PDUs are used to ensure that neighboring routers have
   the same notion of what is the most recent LSP from each other
   router. The sequence number PDUs therefore serve a similar function
   to acknowledgement packets, but allow more efficient operation. There
   are four types of sequence number packets: (i) "Level 1 Complete
   Sequence Numbers PDU"; (ii) "Level 2 Complete Sequence Numbers PDU";
   (iii) "Level 1 Partial Sequence Numbers PDU"; and (iv) "Level 2
   Partial Sequence Numbers PDU". A partial sequence number packet lists
   the most recent sequence number of one or more LSPs, and operates
   much like an acknowlegement. A partial sequence number packet differs
   from an conventional acknowledgement in the sense that it may
   acknowlege multiple LSPs at once, and in the sense that it may act as
   a request for information. A complete sequence number packet contains
   the most recent sequence number of all LSPs in the database. A
   complete sequence number packet may therefore be used to ensure
   synchronization of the database between adjacent routers either
   periodically, or when a link first comes up.

5.2 Overview of IP-Specific Information for IS-IS

   There are six new fields defined for the Integrated IS-IS: (i)
   "Protocols Supported"; (ii) "IP Interface Address"; (iii)
   "Authentication Information"; (iv) "IP Internal Reachability
   Information"; (v) "IP External Reachability Information"; and (vi)
   "Inter-Domain Routing Protocol Information".

   The "Protocols Supported" field identifies the protocols which are



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   supported by each router. This field must be included in all IS-IS
   Hello packets and all LSPs with LSP number 0 transmitted by IP-
   capable routers. If this field is not included in an IS-IS Hello
   packet or an LSP with LSP number 0, it may be assumed that the packet
   was transmitted by an OSI-only router. The "Protocols Supported"
   field must also be included in ISO 9542 ISHs send by IP-capable
   routers over point-to-point links to other IS-IS routers.

   The "IP Interface Address" is included in all IS-IS Hello packets and
   LSPs transmitted by IP-only and dual routers. In the Hello packets,
   this field occurs once only, and contains the IP address(es) of the
   interface on which the Hello packet is transmitted (up to a maximum
   of 63 IP addresses on each interface). If an IS-IS Hello is
   transmitted over an interface which does not have an IP address
   assigned, then this field may be omitted, or may be included with
   zero entries. In Link State Packets, this field contains a list of
   one or more IP addresses corresponding to one or more interfaces of
   the router which originates the LSP. Each IP-capable router must
   include this field in its LSPs. This field may occur multiple times
   in an LSP, and may occur in an LSP with any LSP number.

   The "Authentication Information" field is optional in all IS-IS PDUs.
   If used, it contains information used to authenticate the packet. All
   IS-IS packets (including 9542 IS Hellos) may be authenticated by use
   of this field.

   The "IP Internal Reachability Information" field may be present in
   all LSPs transmitted by IP-capable routers. If present, it identifies
   a list of zero or more [IP address, subnet mask, metrics] reachable
   by the router which originates the LSP. Each entry must contain a
   default metric, and may contain delay, expense, and error metrics. If
   an IP-capable router does not directly reach any IP addresses, then
   it may omit this field, or may include the field with zero [IP
   address, subnet mask, metrics] entries. If included in level 1 LSPs,
   this field includes only entries directly reachable by the router
   which originates the LSP, via one of its interfaces. If included in
   level 2 LSPs, this field includes only entries reachable by the
   router which originates the LSP, either via one of its interfaces, or
   indirectly via level 1 routing. This field may occur multiple times
   in an LSP, and may occur in an LSP with any LSP number.

   The "IP External Reachability Information" field may be present in
   level 2 LSPs transmitted by level 2 IP-capable routers. If present,
   it identifies a list of zero or more [IP address, subnet mask,
   metrics] entries reachable by the router which originates the level 2
   LSP. Each entry must contain a default metric, and may contain delay,
   expense, and error metrics. Each entry may contain metrics of type
   "internal", or of type "external". If a level 2 router does not have



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   any external routes (via neighboring routers in other routing
   domains), when it may omit this field, or may include the field with
   zero entries. This field includes only entries reachable by the
   router which originates the LSP, via a direct link to an external
   router. This field may occur multiple times in a level 2 LSP, and may
   occur in an LSP with any LSP number.

   The "Inter-Domain Routing Protocol Information" field may be present
   in level 2 LSPs transmitted by level 2 IP-capable routers. This field
   is transmitted for the convenience of the external routing protocol,
   and is not used by the IS-IS. For example, this may be used to allow
   border routers to find each other. This field may occur multiple
   times in a level 2 LSP, and may occur in an LSP with any LSP number.

   The DP 10589 version of the OSI IS-IS does not currently allow
   addition of TLV-encoded variable length fields to Sequence Number
   Packets. However, this is being corrected in future versions of
   10589. In addition, this is expected to be the only correction to
   future versions of 10589 that is not backward-compatible with the DP
   version. The Integrated IS-IS therefore makes use of a corrected
   version of DP 10589, such that the encoding of SNPs has been fixed.
   The correct encoding of sequence number packets (as is expected to
   appear in future versions of ISO 10589) is given in Annex B of this
   specification.

   All IP-specific information is encoded in IS-IS packets as variable
   length fields. All variable length fields in IS-IS are encoded as
   follows:

                                         No. of Octets
          +---------------------------+
          |           CODE            |      1
          +---------------------------+
          |          LENGTH           |      1
          +---------------------------+
          |           VALUE           |      LENGTH
          +---------------------------+

        Figure 3 - Encoding of Variable Length Fields

   Any codes in a received PDU that are not recognised shall be ignored
   and, for those packets which are forwarded (specifically Link State
   Packets), passed on unchanged.

   In general, an IS-IS PDU may contain multiple variable length fields,
   some of which contain OSI-specific information (specified in [1]) and
   some of which contain IP-specific information (specified below).
   Except where explicitly stated otherwise, these variable length



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   fields may occur in any order.

5.3 Encoding of IP-Specific Fields in IS-IS PDUs

   This section specifies the detailed encoding of all IP-specific
   fields in IS-IS PDUs. Where a particular field may be present in more
   than one type of PDU, the field is repeated for each type of PDU to
   which it applies.

   Bit and octet numbering is the same as in [1]. In particular, octets
   in a PDU are numbered starting from 1, in increasing order. Bits in
   an octet are numbered from 1 to 8, where bit 1 is the least
   significant bit and is pictured on the right. When consecutive octets
   are used to represent a number, the lower octet number has the most
   significant value.

5.3.1 Level 1 LAN IS to IS Hello PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying

    x CODE - 129

    x LENGTH - total length of the value field (one octet per
      protocol supported).

    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
      each supported data protocol.
                                          No. of Octets
          +---------------------------+
          |           NLPID           |       1
          +---------------------------+
          :                           :
          :                           :
          |---------------------------|
          |           NLPID           |       1
          +---------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 IP Interface Address -- the IP address(es) of the interface
    corresponding to the SNPA over which this PDU is to be transmitted.

    x CODE - 132

    x LENGTH - total length of the value field (four octets per address).



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    x VALUE -
                                          No. of Octets
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
        IP ADDRESS - 4 octet IP Address of the Interface.

  7 Authentication Information -- Information used to authenticate the
    PDU

    x CODE - 133

    x LENGTH - total length of the value field.

    x VALUE - TBD.

5.3.2 Level 2 LAN IS to IS Hello PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying

    x CODE - 129

    x LENGTH  - total length of the value field (one octet per protocol
                supported).

    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
              supported data protocol.
                                          No. of Octets
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 IP Interface Address -- The IP address(es) of the interface



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    corresponding to the SNPA over which this PDU is to be transmitted.

    x CODE - 132

    x LENGTH - total length of the value field (four octets per address).

    x VALUE -
                                     No. of Octets
          +----------------------------+
          |        IP ADDRESS          |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
        IP ADDRESS - 4 octet IP Address of the Interface.

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

5.3.3 Point-to-Point IS to IS Hello PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying

    x CODE - 129

    x LENGTH - total length of the value field (one octet per protocol
               supported).

    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
              supported data protocol.









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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                     No. of Octets
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 IP Interface Address -- The IP address(es) of the interface
    corresponding to the SNPA over which this PDU is to be transmitted.

    x CODE - 132

    x LENGTH - total length of the value field (four octets per address).

    x VALUE -
                                          No. of Octets
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
        IP ADDRESS - 4 octet IP Address of the Interface.

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

5.3.4 Level 1 Link State PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying.

    This must appear once in LSP number 0.



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    x CODE - 129

    x LENGTH - total length of the value field (one octet per protocol
               supported).

    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for each
              supported data protocol.
                                          No. of Octets
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 IP Interface Addresses -- The IP addresss of one or more interfaces
    corresponding to the SNPAs enabled on this Intermediate system
    (i.e., one or more IP addresses of this router).

    This is permitted to appear multiple times, and in an LSP with
    any LSP number.

    x CODE - 132

    x LENGTH - total length of the value field (four octets per address).

    x VALUE -
                                          No. of Octets
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
        IP ADDRESS - 4 octet IP Address

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    x VALUE - TBD

  7 IP Internal Reachability Information -- IP addresses within the
    routing domain reachable directly via one or more interfaces on
    this Intermediate system.

    This is permitted to appear multiple times, and in an LSP with any
    LSP number. However, this field must not appear in pseudonode LSPs.

    x CODE - 128.

    x LENGTH - a multiple of 12.

    x VALUE -
                                          No. of Octets
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+

      DEFAULT METRIC is the value of the default metric for the link
      to the listed neighbor. Bit 8 of this field is reserved, and
      must be set to zero on tranmission and ignored on reception.
      Bit 7 of this field (marked I/E) indicates the metric type



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


      (internal or external) for all four TOS metrics, and must be
      set to zero indicating internal metrics.

      DELAY METRIC is the value of the delay metric for the link to the
      listed neighbor. If this IS does not support this metric it shall
      set the bit "S" to 1 to indicate that the metric is unsupported.
      Bit 7 of this field is reserved, and must be set to zero on
      transmission and ignored on reception.

      EXPENSE METRIC is the value of the expense metric for the link to
      the listed neighbor. If this IS does not support this metric it
      shall set the bit "S" to 1 to indicate that the metric is
      unsupported. Bit 7 of this field is reserved, and must be set to
      zero on transmission and ignored on reception.

      ERROR METRIC is the value of the error metric for the link to
      the listed neighbor. If this IS does not support this metric it
      shall set the bit "S" to 1 to indicate that the metric is
      unsupported. Bit 7 of this field is reserved, and must be set
      to zero on transmission and ignored on reception.

      IP ADDRESS is a 4-octet Internet address

      SUBNET MASK is a 4 octet IP subnet mask.

5.3.5 Level 2 Link State PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying.

    This must appear once in LSP number 0.

    x CODE - 129

    x LENGTH - total length of the value field (one octet per
      protocol supported).












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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
      each supported data protocol.
                                          No. of Octets
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 IP Interface Addresses -- The IP addresss of one or more interfaces
    corresponding to the SNPAs enabled on this Intermediate system
    (i.e., one or more IP addresses of this router).

    This is permitted to appear multiple times, and in an LSP with
    any LSP number. Where a router is both a level 1 and level 2 router,
    it must include the same IP addresses in its level 1 and level 2 LSPs.

    x CODE - 132

    x LENGTH - total length of the value field (four octets per address).

    x VALUE-
                                          No. of Octets
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
        IP ADDRESS - 4 octet IP Address

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

  7 IP Internal Reachability Information -- IP addresses within the
    routing domain reachable directly via one or more interfaces on



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    this Intermediate system.

    This is permitted to appear multiple times, and in an LSP with
    any LSP number. However, this field must not appear in pseudonode
    LSPs.

    x CODE - 128.

    x LENGTH -  a multiple of 12.

    x VALUE -
                                          No. of Octets
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+

      DEFAULT METRIC is the value of the default metric for the link
      to the listed neighbor. Bit 8 of this field is reserved, and must
      be set to zero on transmission and ignored on reception. Bit 7
      of this field indicates the metric type (internal or external)
      for all four TOS metrics, and must be set to zero indicating
      internal metrics.




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


      DELAY METRIC is the value of the delay metric for the link to
      the listed neighbor. If this IS does not support this metric it
      shall set the bit "S" to 1 to indicate that the metric is
      unsupported. Bit 7 of this field is reserved, and must be set
      to zero on transmission and ignored on reception.

      EXPENSE METRIC is the value of the expense metric for the link to
      the listed neighbor. If this IS does not support this metric it
      shall set the bit "S" to 1 to indicate that the metric is
      unsupported. Bit 7 of this field is reserved, and must be set
      to zero on transmission and ignored on reception.

      ERROR METRIC is the value of the error metric for the link to the
      listed neighbor. If this IS does not support this metric it shall
      set the bit "S" to 1 to indicate that the metric is unsupported.
      Bit 7 of this field is reserved, and must be set to zero on
      transmission and ignored on reception.

      IP ADDRESS is a 4-octet Internet address

      SUBNET MASK is a 4 octet IP subnet mask.

  7 IP External Reachability Information -- IP addresses outside the
    routing domain reachable via interfaces on this Intermediate
    system.

    This is permitted to appear multiple times, and in an LSP with
    any LSP number. However, this field must not appear in pseudonode LSPs.

    x CODE - 130.

    x LENGTH - a multiple of 12.

    x VALUE -

















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                          No. of Octets
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          | 0 |I/E|   DEFAULT METRIC   |      1
          +----------------------------+
          | S | R |    DELAY METRIC    |      1
          +----------------------------+
          | S | R |   EXPENSE METRIC   |      1
          +----------------------------+
          | S | R |    ERROR METRIC    |      1
          +----------------------------+
          |         IP ADDRESS         |      4
          +----------------------------+
          |        SUBNET MASK         |      4
          +----------------------------+

      DEFAULT METRIC is the value of the default metric for the
      path to the listed IP addresses. Bit 8 of this field is
      reserved, and must be set to zero on transmission and ignored
      on reception.  Bit 7 of this field indicates the metric type
      (internal or external) for all four TOS metrics, and may be
      set to zero indicating internal metrics, or may be set to 1
      indicating external metrics.

      DELAY METRIC is the value of the delay metric for the path
      to the listed IP addresses. If this IS does not support this
      metric it shall set the bit "S" to 1 to indicate that the metric
      is unsupported. Bit 7 of this field is reserved, and must be
      set to zero on transmission and ignored on reception.

      EXPENSE METRIC is the value of the expense metric for the link
      to the listed IP addresses. If this IS does not support this
      metric it shall set the bit "S" to 1 to indicate that the metric
      is unsupported.  Bit 7 of this field is reserved, and must be



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


      set to zero on transmission and ignored on reception.

      ERROR METRIC is the value of the error metric for the link to
      the listed IP addresses. If this IS does not support this metric
      it shall set the bit "S" to 1 to indicate that the metric is
      unsupported. Bit 7 of this field is reserved, and must be set to
      zero on transmission and ignored on reception.

      IP ADDRESS is a 4-octet Internet address

      SUBNET MASK is a 4 octet IP subnet mask

  7 Inter-Domain Routing Protocol Information -- Inter-domain routing
    protocol information carried transparently through level 2 for
    the convenience of any Inter-Domain protocol that may be running
    in the boundary ISs.

    This is permitted to appear multiple times, and in an LSP with
    any LSP number.

    x CODE - 131.

    x LENGTH - total length of the value field

    x VALUE -
                                             No. of Octets
          +-------------------------------+
          | Inter-Domain Information Type |      1
          +-------------------------------+
          |     External Information      |      VARIABLE
          +-------------------------------+

      INTER-DOMAIN INFORMATION TYPE indicates the type of the
      external information which is encoded in the field.

      EXTERNAL INFORMATION contains inter-domain routing protocol
      information, and is passed transparently by the IS-IS protocol.

5.3.6 Level 1 Complete Sequence Numbers PDU

- Additional codes for IP support are:

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    x VALUE - TBD

5.3.7 Level 2 Complete Sequence Numbers PDU

- Additional codes for IP support are:

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

5.3.8 Level 1 Partial Sequence Numbers PDU

- Additional codes for IP support are:

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

5.3.9 Level 2 Partial Sequence Numbers PDU

- Additional codes for IP support are:

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

5.3.10 ISO 9542 ISH PDU

- Additional codes for IP support are:

  7 Protocols Supported -- the set Network Layer Protocol Identifiers
    for Network Layer protocols that this Intermediate System is
    capable of relaying.



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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


    This appears in ISO 9542 ISH PDUs transmitted on point-to-point
    links.

    x CODE - 129

    x LENGTH - total length of the value field (one octet per
      protocol supported).

    x VALUE - one octet NLPID (as assigned by ISO/TR 9577) for
      each supported data protocol.
                                          No. of Octets
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
          :                            :
          :                            :
          +----------------------------+
          |           NLPID            |      1
          +----------------------------+
        NLPID - ISO/TR 9577 registered Network Layer Protocol Identifier.

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE - TBD

6 Security Considerations

   The integrated IS-IS has a provision for carrying authentication
   information in all IS-IS packets. This is extensible to multiple
   authentication mechanisms. However, currently the only defined
   mechanism is a simple password, transmitted in the clear without
   encryption (see Annex D). The use of a simple password does not
   provide useful protection against intentional misbehavior. Rather,
   this should be thought of as a weak protection against accidental
   errors such as accidental mis-configuration. Definition of other
   authentication mechanisms is beyond the scope of this document.

   Other aspects of security are not discussed in this document.








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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


7 Author's Address

    Ross Callon
    Digital Equipment Corporation
    550 King Street, LKG 1-2/A19
    Littleton, MA 01460-1289
    508-486-5009

8 References

[1]     "Intermediate System to Intermediate System Intra-Domain
        Routeing Exchange Protocol for use in Conjunction with the
        Protocol for Providing the Connectionless-mode Network Service
        (ISO 8473)", ISO DP 10589, February 1990.

[2]     "Protocol for Providing the Connectionless-Mode Network
        Service", ISO 8473, March 1987.

[3]     "End System to Intermediate System Routeing Exchange Protocol
        for Use in Conjunction with the Protocol for Providing the
        Connectionless-Mode Network Service (ISO 8473)", ISO 9542,
        March 1988.

[4]     Braden,R., and Postel,J., "Requirements for Internet Gateways",
        RFC 1009, June 1987.

[5]     Moy,J., "The OSPF Specification", RFC 1131, October 1989.

[6]     Postel,J., "Internetwork Protocol", RFC 791, September 1981.

[7]     Postel,J., "Internet Control Message Protocol", RFC 792,
        September 1981.

[8]     "MIB for Use with the Extended OSI IS-IS in TCP/IP and Dual
        Environments", forthcoming.

[9]     GOSIP Advanced Requirements Group, "Government Open Systems
        Interconnection Profile (GOSIP) Version 2.0 [Final Text]",
        Federal Information Processing Standard, U.S. Department of
        Commerce, National Institute of Standards and Technology,
        Gaithersburg, MD, October 1990.

[10]    "Standard for Local Area Networks and Metropolitan Area
        Networks: Overview and Architecture of Network Standards",
        IEEE Standard 802.1a-1990.






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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                Annex A
               Inter-Domain Routing Protocol Information

   This annex specifies the contents and encoding of the Inter-Domain
   Routing Protocol Information (IDRPI) field. This annex is an integral
   part of the Integrated IS-IS specification. However, it is expected
   that this annex may be augmented or superceded by future efforts
   outside of the scope of the IS-IS specification.

A.1 Inter-Domain Information Type

   As specified in sections 3.4 and 5.3, the IDRPI field consists of a
   one-octet inter-domain information type field, plus a variable
   external information field. This section specifies initial values for
   the inter-domain information type field.  Other values for inter-
   domain information type will be assigned and maintained in future
   versions of the "Assigned Numbers" RFC.

   The following types have been assigned:

        Type = 0        reserved

        Type = 1        local (uses routing-domain specific format)

        Type = 2        AS Number Tag

   Type = 1 indicates that the inter-domain routing protocol information
   uses a format which is local to the routing domain.

   Type = 2 indicates that the inter-domain routing protocol information
   includes autonomous system information used to tag IP external
   reachability information. In this case the inter-domain routing
   protocol information entry must include a single AS number, which is
   used to tag all subsequent External IP Reachability entries until the
   end of the LSP, or until the next occurence of the Inter-Domain
   Routing Protocol Information field.

A.2 Encoding

   As specified in section 5.3.5, the IDPRI entry is encoded as a
   variable length field, as follows:

    x CODE - 131

    x LENGTH - total length of the value field

    x VALUE -




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                             No. of Octets
          +-------------------------------+
          | Inter-Domain Information Type |      1
          +-------------------------------+
          |     External Information      |      VARIABLE
          +-------------------------------+

      INTER-DOMAIN INFORMATION TYPE indicates the type of the
      external information which is encoded in the field.

      EXTERNAL INFORMATION contains inter-domain routing protocol
      information, and is passed transparently by the IS-IS protocol.

   The Inter-domain information type field indicates the type of
   information which is contained in the external information field, as
   follow:

  Type = 0 is reserved (must not be sent, and must be ignored on receipt).

  Type = 1 indicates that the external information field contains
  information which follows a locally specified format.

  Type = 2 indicates that the external information field contains an
  autonomous system number tag, to be applied to subsequent IP external
  reachability information entries. In this case, this "inter-domain
  routing protocol information" entry must contain precisely one 2
  octet AS number. The AS tag is associated with subsequent IP External
  Reachability entries, until the end of the LSP, or until the next
  occurence of the Inter-Domain Routing Protocol Information field.
  In this case, the VALUE contains the following:

    x VALUE -
                                               No. of Octets
          +---------------------------------+
          | Inter-Domain Information Type=2 |      1
          +---------------------------------+
          |   Autonomous System Number      |      2
          +---------------------------------+













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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                Annex B
                  Encoding of Sequence Number Packets

   The Integrated IS-IS protocol defined in this specification makes use
   of the ISO Draft Proposed standard for Intra-domain routing (ISO DP
   10589 [1]) as the base routing protocol, upon which IP support may be
   added.

   However, DP 10589 contains a bug regarding encoding of the variable
   length fields in Sequence Number Packets. In particular, DP 10589
   encodes the variable length fields in SNPs in a manner which is not
   flexible (additional variable length fields cannot be defined for
   sequence number packets), and which is inconsistent with the encoding
   of the variable length fields in all other IS-IS and ES-IS packets.

   The encoding of the variable length fields in SNPs is expected to be
   fixed in future versions of 10589. Also, this bug represents the only
   expected change to 10589 which cannot be made backward compatible
   with existing DP 10589 implementations. For these reasons, the
   current version of the Integrated IS-IS will use the anticipated
   future encoding of the variable length part of the SNPs. This should
   allow future versions of this specification to be compatible with
   implementations based on this specification.

   This annex specifies the encoding of SNPs, as amended to fix the
   encoding of variable length fields. This annex is an integral part of
   the Integrated IS-IS specification.

   The encoding of SNPs for OSI-only use is shown in this section. For
   IP-only or Integrated use, the additional variable length fields
   specified in sections 5.3.6 through 5.3.9 are also applicable to
   SNPs.



















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


B.1 Level 1 Complete Sequence Numbers PDU

                                              No. of Octets
          +--------------------------------+
          |     INTRA-DOMAIN ROUTEING      |      1
          |     PROTOCOL DISCRIMINATOR     |
          +--------------------------------+
          |        LENGTH INDICATOR        |      1
          +--------------------------------+
          |    VERSION/PROTOCOL ID EXT     |      1
          +--------------------------------+
          |            RESERVED            |      1
          +--------------------------------+
          | R | R | R |        TYPE        |      1
          +--------------------------------+
          |            VERSION             |      1
          +--------------------------------+
          |              ECO               |      1
          +--------------------------------+
          |            USER ECO            |      1
          +--------------------------------+
          |           PDU LENGTH           |      2
          +--------------------------------+
          |           SOURCE ID            |      7
          +--------------------------------+
          |          START LSP ID          |      8
          +--------------------------------+
          |           END LSP ID           |      8
          +================================+====================
          |     VARIABLE LENGTH FIELDS     |      VARIABLE
          +--------------------------------+


- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 24. Note bits 6, 7 and 8 are Reserved,
  which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)
  generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this
  Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this
  Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                              No. of Octets
          +--------------------------------+
          |              CODE              |      1
          +--------------------------------+
          |             LENGTH             |      1
          +--------------------------------+
          |             VALUE              |      LENGTH
          +--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

  7 LSP Entries -- This may appear multiple times. The option fields,
    if they appear more than once, shall appear sorted into ascending
    LSPID order.

    x CODE - 9

    x LENGTH - total length of the value field.

    x VALUE - a list of LSP entries of the form:














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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990



                                              No. of Octets
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+
          :                                :
          :                                :
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).


















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


B.2 Level 2 Complete Sequence Numbers PDU

                                              No. of Octets
          +--------------------------------+
          |     INTRA-DOMAIN ROUTEING      |      1
          |     PROTOCOL DISCRIMINATOR     |
          +--------------------------------+
          |        LENGTH INDICATOR        |      1
          +--------------------------------+
          |    VERSION/PROTOCOL ID EXT     |      1
          +--------------------------------+
          |            RESERVED            |      1
          +--------------------------------+
          | R | R | R |        TYPE        |      1
          +--------------------------------+
          |            VERSION             |      1
          +--------------------------------+
          |              ECO               |      1
          +--------------------------------+
          |            USER ECO            |      1
          +--------------------------------+
          |           PDU LENGTH           |      2
          +--------------------------------+
          |           SOURCE ID            |      7
          +--------------------------------+
          |          START LSP ID          |      8
          +--------------------------------+
          |           END LSP ID           |      8
          +================================+====================
          |     VARIABLE LENGTH FIELDS     |      VARIABLE
          +--------------------------------+


- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (33.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 25. Note bits 6, 7 and 8 are Reserved,
  which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt




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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate System (with zero Circuit ID)
  generating this Sequence Numbers PDU.

- START LSP ID - 8 octet ID of first LSP in the range covered by this
  Complete Sequence Numbers PDU.

- END LSP ID - 8 octet ID of last LSP in the range covered by this
  Complete Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                              No. of Octets
          +--------------------------------+
          |              CODE              |      1
          +--------------------------------+
          |             LENGTH             |      1
          +--------------------------------+
          |             VALUE              |      LENGTH
          +--------------------------------+

Any codes in a received CSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- this may appear multiple times. The option fields,
  if they appear more than once, shall appear sorted into ascending
  LSPID order.

  x CODE - 9

  x LENGTH - total length of the value field.

  x VALUE - a list of LSP entries of the form:














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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                              No. of Octets
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+
          :                                :
          :                                :
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).



















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


B.3 Level 1 Partial Sequence Numbers PDU

                                              No. of Octets
          +--------------------------------+
          |     INTRA-DOMAIN ROUTEING      |      1
          |     PROTOCOL DISCRIMINATOR     |
          +--------------------------------+
          |        LENGTH INDICATOR        |      1
          +--------------------------------+
          |    VERSION/PROTOCOL ID EXT     |      1
          +--------------------------------+
          |            RESERVED            |      1
          +--------------------------------+
          | R | R | R |        TYPE        |      1
          +--------------------------------+
          |            VERSION             |      1
          +--------------------------------+
          |              ECO               |      1
          +--------------------------------+
          |            USER ECO            |      1
          +--------------------------------+
          |           PDU LENGTH           |      2
          +--------------------------------+
          |           SOURCE ID            |      7
          +================================+====================
          |     VARIABLE LENGTH FIELDS     |      VARIABLE
          +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5)  26. Note bits 6, 7 and 8 are Reserved,
  which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)



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  generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                              No. of Octets
          +--------------------------------+
          |              CODE              |      1
          +--------------------------------+
          |             LENGTH             |      1
          +--------------------------------+
          |             VALUE              |      LENGTH
          +--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7  LSP Entries - this may appear multiple times. The option fields,
   if they appear more than once, shall appear sorted into ascending
   LSPID order.

   x CODE - 9

   x LENGTH - total length of the value field.



























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   x VALUE - a list of LSP entries of the form:

                                              No. of Octets
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+
          :                                :
          :                                :
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER - Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP number
octet of the LSPID is the least significant octet).

















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


B.4 Level 2 Partial Sequence Numbers PDU

                                              No. of Octets
          +--------------------------------+
          |     INTRA-DOMAIN ROUTEING      |      1
          |     PROTOCOL DISCRIMINATOR     |
          +--------------------------------+
          |        LENGTH INDICATOR        |      1
          +--------------------------------+
          |    VERSION/PROTOCOL ID EXT     |      1
          +--------------------------------+
          |            RESERVED            |      1
          +--------------------------------+
          | R | R | R |        TYPE        |      1
          +--------------------------------+
          |            VERSION             |      1
          +--------------------------------+
          |              ECO               |      1
          +--------------------------------+
          |            USER ECO            |      1
          +--------------------------------+
          |           PDU LENGTH           |      2
          +--------------------------------+
          |           SOURCE ID            |      7
          +================================+====================
          |    VARIABLE LENGTH FIELDS      |      VARIABLE
          +--------------------------------+

- INTRADOMAIN ROUTEING PROTOCOL DISCRIMINATOR - architectural constant

- LENGTH INDICATOR - Header Length in octets (17.)

- VERSION/PROTOCOL ID EXTENSION - 1

- RESERVED - transmitted as 0, ignored on receipt

- TYPE (bits 1 through 5) - 27. Note bits 6, 7 and 8 are Reserved,
  which means they are transmitted as 0 and ignored on receipt.

- VERSION - 1

- ECO - transmitted as zero, ignored on receipt

- USER ECO - transmitted as zero, ignored on receipt

- PDU LENGTH - Entire Length of this PDU, in octets, including header

- SOURCE ID - 7 octet ID of Intermediate system (with zero Circuit ID)



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  generating this Sequence Numbers PDU.

- VARIABLE LENGTH FIELDS - fields of the form:

                                              No. of Octets
          +--------------------------------+
          |              CODE              |      1
          +--------------------------------+
          |             LENGTH             |      1
          +--------------------------------+
          |             VALUE              |      LENGTH
          +--------------------------------+

Any codes in a received PSNP that are not recognised are ignored.

Currently defined codes are:

7 LSP Entries -- this may appear multiple times. The option fields,
  if they appear more than once, shall appear sorted into ascending
  LSPID order.

  x CODE - 9

  x LENGTH - total length of the value field.

  x VALUE - a list of LSP entries of the form:

























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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                              No. of Octets
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+
          :                                :
          :                                :
          +--------------------------------+
          |       REMAINING LIFETIME       |      2
          +--------------------------------+
          |             LSP ID             |      8
          +--------------------------------+
          |         LSP SEQ NUMBER         |      4
          +--------------------------------+
          |            CHECKSUM            |      2
          +--------------------------------+

7 REMAINING LIFETIME - Remaining Lifetime of LSP.

7 LSP ID - 8 octet ID of the LSP to which this entry refers.

7 LSP SEQ NUMBER  -Sequence number of LSP.

7 CHECKSUM - Checksum reported in LSP.

The entries shall be sorted into ascending LSPID order (the LSP
number octet of the LSPID is the least significant octet).



















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RFC 1195         OSI ISIS for IP and Dual Environments     December 1990


                                Annex C
                  Dijkstra Calculation and Forwarding

   Annex C.2 of ISO DP 10589 [1] specifies the SPF (Dikskstra) algorithm
   for calculating routes with the IS-IS routing protocol. This annex
   specifies modifications to the SPF algorithm for supporting IP and
   dual routing, and specifies a compatible method for forwarding IP
   packets. This will result in an order of preference of routes which
   is compatible with that specified in section 3.10.

   This annex is included for informational purposes.

C.1 SPF Algorithm for IP and Dual Use

   This section specifies an SPF Algorithm for calculating routes with
   the IS-IS routing protocol, for support of both TCP/IP and OSI. This
   is based on an extention to the algorithm specified in annex C.2 of
   ISO DP 10589 [1].

   An algorithm invented by Dijkstra known as shortest path first (SPF)
   is used as the basis for the route calculation. It has a
   computational complexity of the square of the number of nodes, which
   can be decreased to the number of links in the domain times the log
   of the number of nodes for sparse networks (networks which are not
   highly connected).

   A number of additional optimizations are possible:

   1) If the routing metric is defined over a small finite field (as in
      this standard), the factor of log n may be removed by using data
      structures which maintain a separate list of systems for each value
      of the metric rather than sorting the systems by logical distance.

   2) Updates can be performed incrementally without requiring a complete
      recalculation. However, a full update must be done periodically to
      ensure recovery from data corruption, and studies suggest that with
      a very small number of link changes (perhaps 2) the expected
      computation complexity of the incremental update exceeds the
      complete recalculation. Thus, this annex specifies the algorithm
      only for the full update.

   3) If only End System LSP information has changed, it is not necessary
      to re-compute the entire Dijkstra tree. If the proper data
      structures are used, End Systems (including IP reachability
      entries) may be attached and detached as leaves of the tree and
      their forwarding information base entries altered as appropriate.

   The original SPF algorithm does not support load splitting over



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   multiple paths. The algorithm in this annex does permit load
   splitting by identifying a set of equal cost paths to each
   destination rather than a single least cost path.

C.1.1 Databases

  PATHS -- This represents an acyclic directed graph of shortest paths
  from the system S performing the calculation. It is stored as a set
  of triples of the form <N,d(N),{Adj(N)}>, where:

      N is a system identifier. In the level 1 algorithm, N is a
      6 octet ID for OSI end systems, a 7 octet ID for routers, or
      an 8 octet IP Internal Reachability Information entry. For a
      router which is not a pseudonode, it is the 6 octet system ID,
      with a 0 appended octet. For a pseudonode it is a true 7 octet
      quantity, comprised of the 6 octet Designated Intermediate
      System ID and the extra octet assigned by the Destinated Router.
      The IP Internal Reachability Information entries consist of a
      4 octet IP address plus a 4 octet subnet mask, and will always
      be a leaf, i.e., "End System" in PATHS.

      In the level 2 algorithm, N is either a 7 octet router or
      pseudonode ID (as in the level 1 algorithm); a variable
      length OSI address prefix; an 8 octet IP Internal Reachability
      Information Entry, or an 8 octet IP External Reachability
      Information entry. The variable length OSI address prefixes,
      and 8 octet IP Reachability Information entries will always
      be a leaf, i.e., "End System" in PATHS. As above, the IP
      Reachability Information entries consist of an [IP address,
      subnet mask] combination.

      d(N) is N's distance from S (i.e., the total metric value
      from N to S).

      {Adj(N)} is a set of valid adjacencies that S may use for
      forwarding to N.

   When a system is placed on PATHS, the path(s) designated by its
   position in the graph is guaranteed to be a shortest path.

  TENT -- This is a list of triples of the form <N,d(N),{Adj(N)}>,
  where N, d(N), and {Adj(N)} are as defined above for PATHS.

  TENT can intuitively be thought of as a tentative placement
  of a system in PATHS. In other words, the triple <N,x,{A}>
  in TENT means that if N were placed in PATHS, d(N) would be x,
  but N cannot be placed on PATHS until is is guaranteed that
  no path shorter than x exists.



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  Similarly, the triple <N,x,{A,B}> in TENT means that if N
  were placed in PATHS, then d(N) would be x via either
  adjacency A or B.

   Note: It is suggested that the implementation maintain the database
   TENT as a set of list of triples of the form <*,Dist,*>, sorted by
   distance Dist. In addition, it is necessary to be able to process
   those systems which are pseudonodes before any non-pseudonodes at the
   same distance Dist.

   The 8 octet system identifiers which specify IP reachability entries
   must always be distinguishable from other system identifiers. As
   specified in section 3.10, two IP reachability entries which differ
   only in the subnet mask are still considered to be separate, and will
   therefore have distinct system identifiers N. The SPF algorithm will
   therefore calculate routes to each such entry, and the correct entry
   will be selected in the forwarding process.

C.1.2 Use of Metrics in the SPF Algorithm

   Internal metrics are not comparable to external metrics. For external
   routes (routes to destinations outside of the routing domain), the
   cost d(N) of the path from N to S may include both internal and
   external metrics. d(N) may therefore be maintained as a two-
   dimensioned vector quantity (specifying internal and external metric
   values).

   d(N) is initialized to [internal metric = 0, external metric = 0].

   In incrementing d(N) by 1, if the internal metric value is less than
   the maximum value MaxPathMetric, then the internal metric value is
   incremented by one and the external metric value left unchanged; if
   the internal metric value is equal to the maximum value
   MaxPathMetric, then the internal metric value is set to 0 and the
   external metric value is incremented by 1. Note that this can be
   implemented in a straightforward manner by maintaining the external
   metric as the high order bits of the distance.

   In the code of the algorithm below, the current path length is held
   in the variable "tentlength". This variable is a two-dimensional
   quantity tentlength=[internal metric, external metric], and is used
   for comparing the current path length with d(N) as described above.
   Tentlength is incremented in the same manner as d(N).

C.1.3 Overview of the Algorithm

   The basic algorithm, which builds PATHS from scratch, starts out by
   putting the system doing the computation on PATHS (no shorter path to



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   SELF can possibly exist). TENT is then pre-loaded from the local
   adjacency database.

   Note that a system is not placed on PATHS unless no shorter path to
   that system exists. When a system N is placed on PATHS, the path to
   each neighbor M of N, through N, is examined, as the path to N plus
   the link from N to M. If <M,*,*> is in PATHS, this new path will be
   longer, and thus ignored.

   If <M,*,*> is in TENT, and the new path is shorter, the old entry is
   removed from TENT and the new path is placed in TENT. If the new path
   is the same length as the one in TENT, then the set of potential
   adjacencies {Adj(M)} is set to the union of the old set (in TENT) and
   the new set {Adj(N)}. If M is not in TENT, then the path is added to
   TENT.

   Next the algorithm finds the triple <N,x,{Adj(N)}> in TENT, with
   minimal x. Note: This is done efficiently because of the optimization
   described above. When the list of triples for distance Dist is
   exhausted, the algorithm then increments Dist until it finds a list
   with a triple of the form <*,Dist,*>.

   N is placed in PATHS. We know that no path to N can be shorter than x
   at this point because all paths through systems already in PATHS have
   already been considered, and paths through systems in TENT still have
   to be greater than x because x is minimal in TENT.

   When TENT is empty, PATHS is complete.

   Note that external metrics can only occur in "IP External
   Reachability Information" entries, which correspond to a leaf (i.e.,
   End System in PATHS). Any route utilizing an entry with an external
   metric will always be considered to be less desireable than any entry
   which uses an internal metric. This implies that in the addition of
   systems to PATHS, all systems reachable via internal routes are
   always added before any system reachable via external routes.

C.1.4 The Algorithm

   The Decision Process Algorithm must be run once for each supported
   routing metric (i.e., for each supported Type of Service). A level 1
   router runs the algorithm using the level 1 LSP database to compute
   level 1 paths (for those level 1 routers which are not level 2
   routers, this includes the path to the nearest attached level 2
   router). Level 2 routers also separately run the algorithm using the
   level 2 LSP database to compute level 2 paths. IP-capable level 2
   routers must keep level 2 internal IP routes separate from level 2
   external IP routes.



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   Note that this implies that routers which are both level 1 and level
   2 routers, and which support all four routing metrics, must run the
   SPF algorithm 8 times (assuming partition repair is not implemented).

   If this system is a Level 2 Router which supports the partition
   repair optional function the Decision Process algorithm for computing
   Level 1 paths must be run twice for the default metric. This first
   execution is done to determine which of the area's
   manualAreaAddresses are reachable in this partition, and to elect a
   Partition Designated Level 2 Router for the partition. The partition
   Designated Level 2 Router will determine if the area is partitioned
   and will create virtual Level 1 links to the other Partition
   Designated Level 2 Routers in the area in order to repair the Level 1
   partition. This is further described in section 7.2.10 of [1].

   The SPF algorithm specified here will calculate routes for both OSI
   and IP. In particular, routes are calculated to all system
   identifiers N, where N may specify an OSI End System, the OSI address
   of a router, or an IP reachability entry. In computing the forwarding
   database, it is an implementation specific issue whether the IP
   forwarding database is kept separately from the OSI forwarding
   database. Where appropriate, this annex will refer separately to
   entries in these two forwarding data bases. This is not meant to
   preclude any specific implementation method.

   OSI and IP use separate mechanisms to determine whether a packet is
   in the area (in particular, OSI makes use of area addresses, and IP
   determines that a destination is not in an area by looking in the
   level 1 forwarding database and determining that no entry exists for
   that destination within the area). The route to the nearest level 2
   router will result in separate entries in the forwarding database for
   OSI and IP. For IP, the route to the nearest attached level 2 router
   may be entered in the forwarding database as a default route (i.e., a
   route with a subnet mask of all 0).

   One approach would be to put the results of each Dijkstra algorithm
   in a separate forwarding database. For a router which supports both
   level 1 and level 2 routing (including level 2 internal and level 2
   external routes), and which supports all four types of service, this
   would result in twelve separate forwarding databases for IP.
   Implementations may choose to minimize the number of forwarding
   databases by combining the information from the multiple Dijkstra
   calculations into a single database per supported TOS. This is
   discussed in section C.2 below.

   The SPF algorithm specified in section C.2.3 of [1] is amended to
   appear as follows:




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   Step 0: Initialize TENT and PATHS to empty. Initialize tentlength to
   [internalmetric=0, externalmetric=0].

   (tentlength is the pathlength of elements in TENT that we are
   examining.)

   1) Add <SELF,0,W> to PATHS, where W is a special value indicating
      traffic to SELF is passed up to internal processes (rather than
      forwarded).

   2) Now pre-load TENT with the local adjacency database (Each
      entry made to TENT must be marked as being either an End System
      or a router to enable the check at the end of Step 2 to be made
      correctly - Note that each local IP reachability entry is
      included as an adjacency, and is marked as being an End System).
      For each adjacency Adj(N) (including level 1 OSI Manual
      Adjacencies, or level 2 OSI enabled reachable addresses, and
      IP reachability entries) on enabled circuits, to system N of
      SELF in state "Up" compute:

         d(N) = cost of the parent circuit of the adjacency (N),
         obtained from metric.k , where k = one of {default metric,
         delay metric, monetary metric, error metric}

         Adj(N) = the adjacency number of the adjacency to N

   3) If a triple <N,x,{Adj(M)}> is in TENT, then:

         If x = d(N), then {Adj(M)} <--- {Adj(M)} U {Adj(N)}.

   4) If N is a router or an OSI End System entry, and there are now
      more adjacencies in {Adj(M)} than maximumPathSplits, then remove
      excess adjacencies as described in Clause 7.2.7 of [1]. If N
      is an IP Reachability Entry, then excess adjacencies may be
      removed as desired. This will not effect the correctness of
      routing, but may eliminate the determinism for IP routes (i.e.,
      IP packets still follow optimal routes within an area, but
      where multiple equally good routes exist, will not necessarily
      follow precisely the route that any one particular router
      would have anticipated).

   5) If x < d(N), do nothing.

   6) If x > d(N), remove <N,x,{Adj(M)}> from TENT and add the triple
      <N,d(N),{Adj(N)}>.

   7) If no triple <N,x,{Adj(M)}> is in TENT, then add <N,d(N),{Adj(N)}>
      to TENT.



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   8) Now add systems to which the local router does not have adjacencies,
      but which are mentioned in neighboring pseudonode LSPs. The
      adjacency for such systems is set to that of the designated router.
      Note that this does not include IP reachability entries from
      neighboring pseudonode LSPs. In particular, the pseudonode LSPs
      do not include IP reachability entries.

   9) For all broadcast circuits in state "On", find the pseudonode
      LSP for that circuit (specifically, the LSP with number zero and
      with the first 7 octets of LSPID equal to LnCircuitID for that
      circuit, where n is 1 (for level 1 routing) or 2 (level 2
      routing)). If it is present, for all the neighbors N reported in
      all the LSPs of this pseudonode which do not exist in TENT add
      an entry <N,d(N),{Adj(N)}> to TENT, where:

         d(N) = metric.k  of the circuit.
         Adj(N) = the adjacency number of the adjacency to the DR.

   10) Go to Step 2.

   Step 1: Examine the zeroeth link state PDU of P, the system just
   placed on PATHS (i.e., the LSP with the same first 7 octets of LSPID
   as P, and LSP number zero).

   1) If this LSP is present, and the "Infinite Hippity Cost" bit is
      clear, then for each LSP of P (i.e., all LSPs with the same
      first 7 octets of LSPID and P, irrespective of the value of
      LSP number) compute:

         dist(P,N) = d(P) + metric.k(P,N)

   for each neighbor N (both End System and router) of the system P. If
   the "Infinite Hippity Cost" bit is set, only consider the End System
   neighbors of the system P. Note that the End Systems neighbors of the
   system P includes IP reachable address entries included in the LSPs
   from system P. Here, d(P) is the second element of the triple

         <P,d(P),{Adj(P)}>

   and metric.k(P,N) is the cost of the link from P to N as reported in
   P's link state PDU.

   2) If dist(P,N) > MaxPathMetric, then do nothing.

   3) If <N,d(N),{Adj(N)}> is in PATHS, then do nothing.

         Note: d(N) must be less than dist(P,N), or else N would not
         have been put into PATHS. An additional sanity check may be



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         done here to ensure that d(N) is in fact less than dist(P,N)

   4) If a triple <N,x,{Adj(N)}> is in TENT, then:

     a) If x = dist(P,N), then {Adj(N)} <-- {Adj(N)} U {Adj(P)}.

     b) If N is a router or an OSI end system, and there are now more
        adjacencies in {Adj(N)} than maximumPath Splits, then remove
        excess adjacencies, as described in clause 7.2.7 of [1]. For
        IP Reachability Entries, excess adjacencies may be removed as
        desired. This will not effect the correctness of routing, but
        may eliminate the determinism for IP routes (i.e., IP packets
        will still follow optimal routes within an area, but where
        multiple equally good routes exist, will not necessarily follow
        precisely the route that any one particular router would have
        anticipated).

     c) if x < dist(P,N), do nothing.

     d) if x > dist(P,N), remove <N,x,{Adj(N)}> from TENT, and add
        <N,dist(P,N),{Adj(P)}>

   5) if no triple <N,x,{Adj(N)}> is in TENT, then add
      <N,dist(P,N),{Adj(P)}> to TENT.

   Step 2: If TENT is empty, stop. Else:

   1) Find the element <P,x,{Adj(P)}>, with minimal x as follows:

     a) If an element <*,tentlength,*> remains in TENT in the list for
        tentlength, choose that element. If there are more than one
        elements in the list for tentlength, choose one of the elements
        (if any) for a system which is a pseudonode in preference to one
        for a non-pseudonode. If there are no more elements in the list
        for tentlength, increment tentlength and repeat Step 2.

     b) Remove <P,tentlength,{Adj(P)}> from TENT.

     c) Add <P,d(P),{Adj(P)}> to PATHS.

     d) If this is the Level 2 Decision Process running, and the system
        just added to PATHS listed itself as Partition Designated Level 2
        Intermediate system, then additionally add <AREA.P,d(P),{Adj(P)}>
        to PATHS, where AREA.P is the Network Entity Title of the other
        end of the Virtual Link, obtained by taking the first AREA
        listed in P's LSP and appending P's ID.

     e) If the system just added to PATHS was an end system, go to



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        step 2. Else go to Step 1.

   NOTE - In the level 2 context, the "End Systems" are the set of
   Reachable Address Prefixes (for OSI), the set of Area Addresses with
   zero cost (again, for OSI), plus the set of IP reachability entries
   (including both internal and external).

C.2 Forwarding of IP packets

   The SPF algorithm specified in section C.1 may be used to calculate
   (logically) separate IP forwarding tables for each type of service,
   and for level 1, level 2 internal, and level 2 external routes.
   Section C.2.1 describes how to forward IP packets, based on these
   multiple forwarding databases. Section C.2.2 describes how the
   multiple forwarding databases can be combined into a single
   forwarding database per supported TOS.

C.2.1 Basic Method for Forwarding IP packets

   For level 1-only routers:

   - Determine if the IP destination address matches any entry in the
     level 1 forwarding table for the specified TOS.

   - Determine if the IP destination address matches any entry in the
     level 1 forwarding table for the default TOS.

   - If default TOS resulted in more specific entry, forward according
     to default TOS.

   - If equally specific entries found, or specified TOS resulted in
     more specific entry, forward according to specified TOS

   - If no entry was found (which includes no default route entry), then
     destination is unreachable.

   Note: For level 1 only routers, the route to the nearest attached
   level 2 router will be entered into the forwarding database as a
   default route (i.e., a route with a subnet mask which is all 0). Thus
   this last event (no entry found) can occur only if there is no
   attached level 2 router reachable in the area.

   For routers which are both level 1 and level 2 routers:

   - Determine if the IP destination address matches any entry in the
     level 1 forwarding table for the specified TOS.

   - Determine if the IP destination address matches any entry in the



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     level 1 forwarding table for the default TOS.

   - If default TOS resulted in more specific entry (i.e., more bits in
     the subnet mask take the value 1), forward according to default TOS.

   - If equally specific entries found, or specified TOS resulted in
     more specific entry, forward according to specified TOS

   - If no entry found:

   - Determine if the IP destination address matches any entry in the
     level 2 internal forwarding table for the specified TOS.

   - Determine if the IP destination address matches any entry in the
     level 2 internal forwarding table for the default TOS.

   - If default TOS resulted in more specific entry, forward according
     to default TOS.

   - If equally specific entries found, or specified TOS resulted in
     more specific entry, forward according to specified TOS

   - If no entry found:

   - Determine if the IP destination address matches any entry in the
     level 2 external forwarding table for the specified TOS.

   - Determine if the IP destination address matches any entry in the
     level 2 external forwarding table for the default TOS.

   - If default TOS resulted in more specific entry, forward according
     to default TOS.

   - If equally specific entries found, or specified TOS resulted in
     more specific entry, forward according to specified TOS

   - If no entry is found, then destination is unreachable

   For level 2-only routers, the above algorithm can be used, except
   since there is no level 1 forwarding database, the corresponding
   steps can be skipped.

   As discussed in section 3.10.2, for level 2 routers which are
   announcing manually configured summary addresses in their level 2
   LSPs, in some cases there will exist IP addresses which match the
   manually configured addresses, but which do not match any addresses
   which are reachable via level 1 routing in the area. Packets to such
   addresses are handled according to the rules specified in section



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   3.10.2. This may be accomplished by adding the manually configured
   [IP address, subnet mask] entry to the level 2 forwarding database
   (for the appropriate TOS), with a special "next hop" address which
   specifies that packets for which this entry is selected are to be
   discarded. This will work correctly because more desireable entries
   (such as delivering the packet via level 1 routing to the correct
   destination, or a more specific level 2 route) will automatically
   take precedence according to the forwarding rules specified above.
   Less desireable routes (such as using a level 2 external route to the
   "default route" entry) are not possible because other level 2 routers
   will believe the summary addresses advertised by this router.

C.2.2 Reduction of IP Forwarding Databases

   The multiple forwarding databases used in the basic forwarding method
   in section C.2.1 can be reduced, by combining the multiple databases
   into one database for each supported TOS.

   For reduction of IP forwarding databases, it is assumed that for any
   two overlapping address entries, either the entries are identical, or
   one range contains the other. In other words, for any two [IP
   address, subnet mask] entries A and B, if there is at least one IP
   address which matches both entries, then either: (i) the two entries
   are identical; or (ii) entry A contains entry B (i.e., any IP address
   which matches entry B also matches entry A); or (iii) entry B
   contains entry A (any IP address which matches entry A also matches
   entry B).

   Non-contiguous subnet masks can be configured to violate this
   assumption. For example, consider the two entries:

   - A=[address="01010101 00000101 00000000 00000000",
     mask="11111111 00001111 00000000 00000000"]

   - B=[address="01010101 01010000 00000000 00000000",
     mask="11111111 11110000 00000000 00000000"]

   In this case neither entry contains the other. Specifically;

   - there are IP addresses which match both A and B (e.g.,
     "01010101 01010101 xxxxxxxx xxxxxxxx"),

   - there are IP addresses which match A but not B (e.g.,
     "01010101 11110101 xxxxxxxx xxxxxxxx")

   - there are IP addresses which match B but not A (e.g.,
     "01010101 01011111 xxxxxxxx xxxxxxxx").




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   The reduction of the multiple forwarding databases for each TOS to a
   single database for each TOS is based on the use of "best match"
   routing, combined with reduction of the entries placed in the
   forwarding database in order to eliminate entries which are not to be
   selected (based on the order of preference of routes specified in
   section 3.10). The specific algorithm for creation of the IP
   forwarding database can be described as follows:

   1) Make use of the the Dijkstra algorithm described in section C.1 to
      create separate forwarding databases for each supported TOS for
      level 1 routes, level 2 internal routes, and level 2 external
      routes. (Note that each entry in the forwarding database will
      specify an [IP address, subnet mask] combination, as well as the
      next hop router for IP packets which match that entry).

   2) For each level 1 route entry which has been placed in the level 1
      IP forwarding database for a specific TOS, copy that entry into
      the overall IP forwarding database for that TOS.

   3) For each route entry X which has been placed in the level 2 internal
      IP forwarding database for a specific TOS, search for overlapping
      entries in the level 1 IP forwarding database for the specific TOS,
      and also for the default TOS:

      a) If there is any overlapping entry Y in the level 1 forwarding
         database (for the specfic TOS, or for the default TOS) such
         that either (i) Y contains X; or (ii) Y is identically specific
         to X; then ignore entry X.

      b) Otherwise, copy entry X into the overall IP forwarding database
         for the specific TOS.

   4) For each route entry X which has been placed in the level 2
      external IP forwarding database for a specific TOS, search for
      overlapping entries in the level 1 IP forwarding database for
      the specific TOS, and for the default TOS, and the level 2
      internal IP forwarding database for the specific TOS, and for
      the default TOS.

      a) If there is an overlapping entry Y such that either (i) Y
         contains X; or (ii) Y is identically specific to X; then
         ignore entry X.

      b) Otherwise, copy entry X into the overall IP forwarding database
         for the specific TOS.

   This method will result in one forwarding database for each supported
   TOS. The forwarding of packets can then be simplified to be as follows:



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   1) For IP packets which map to the default TOS metric (or to an
      unsupported TOS metric), search the default TOS forwarding
      database and select the entry which has the most specific match.
      Forward the packet accordingly.

   2) For packets which map to a specific (non-default) TOS metric,
      search the specific TOS forwarding database and select the entry
      j which has the most specific match. Also search the default TOS
      forwarding database and select the entry k which has the most
      specific match. Forward the packet as follows:

      a) If k is more specific than j, forward according to entry k

      b) If j and k are equally specific, forward according to entry j

      c) If j is more specific than k, forward according to entry j



































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                                  Annex D
                      Use of the Authentication Field

   The use of the Authentication field is outside of the scope of this
   specification. However, there is a urgent need for simple error
   detection/authentication mechanisms (such as a simple password) to
   protect against certain types of errors. This annex therefore
   proposes a possible use of this field.

   This annex is included for informational purposes.

D.1 Authentication Field in IS-IS packets

   All IS-IS packets may optionally include the authentication field, as
   described in sections 3.9 and 5 of this specification. As described
   in section 5, the authentication field is encoded as a (Code, Length,
   Value) triplet. This annex proposes that the contents of the Value
   field consist of a one octet "Authentication Type" field, plus a
   variable length "Authentication Information" field. A specific value
   of the "Authentication Type" is assigned to passwords, transmitted in
   the clear without encryption. The authentication field is encoded as
   follows:

  7 Authentication Information -- Information used to authenticate
    the PDU

    x CODE - 133

    x LENGTH - total length of the value field

    x VALUE -
                                              No. of Octets
          +--------------------------------+
          |      Authentication Type       |      1
          +--------------------------------+
          |   Authentication Information   |      VARIABLE
          +--------------------------------+

The Authentication Type is assigned as follows:

      Type  =  0        reserved

      Type  =  1        simple password

      Type  >  1        reserved






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D.2 Authentication Type 1 - Simple Password

   Using this authentication type, a variable length password is passed
   in the clear (i.e., not encrypted) in the Authentication Information
   field.

   WARNING: The use of a simple password does not provide useful
   protection against intentional misbehavior. In particular, since the
   password is transmitted in the clear without encryption, it is easy
   for a hostile system to intercept the passwords, and to transmit
   authenticated packets. The use of simple passwords should be
   considered only as a weak protection against accidental errors such
   as accidental misconfiguration.

   The password shall be configured on a per-link, per-area, and per-
   domain basis. Specifically, when this form of authentication is used:

   - IS-IS Hello and 9542 IS Hello packets shall contain the
     per-link password

   - Level 1 Link State Packets shall contain the per-area password

   - Level 2 Link State Packets shall contain the per-domain password

   - Level 1 Sequence Number Packets shall contain the per-area password

   - Level 2 Sequence Number Packets shall contain the per-domain
     password

   Also, each of these three passwords shall be configured with: (i)
   "Transmit Password", whose value is a single password, and (ii)
   "Receive Passwords", whose value is a set of passwords. The transmit
   password value is always transmitted. However, any password contained
   in the receive password set will be accepted on receipt. This method
   allows the graceful changing of passwords without temporary loss of
   connectivity.

   For example, consider the case that an area has the configured area
   password "OLDAREAPASSWORD". In this case, the per-area transmit
   password value is set to OLDAREAPASSWORD, and the per-area receive
   password value is set to {OLDAREAPASSWORD}. Suppose that it is
   desired to change the per-area password to "NEWERPASSWORD".  The
   first step would be to manually configure all of the routers in the
   area to set the per-area receive password value to {OLDAREAPASSWORD,
   NEWERPASSWORD}. When this step is complete, then all routers in the
   area will still be using the old password OLDAREAPASSWORD in their
   level 1 LSPs and SNPs. However, they will also accept the alternate
   password NEWERPASSWORD. The second step would be to configure the



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   routers in the area to set the per-area transmit password to
   NEWERPASSWORD. When the second step is complete, then all routers
   will be using the new value of the per-area password, but will accept
   the old value as well. Finally, the third step is to change all
   routers in the area to have the per-area receive password set to
   {NEWERPASSWORD}.

   By configuring transmit and receive values for the passwords in this
   manner, it is possible to maintain continuous correct operation. For
   example, in the middle of the second step in the above example, some
   of the routers in the area will be transmitting level 1 LSPs and SNPs
   using the old password OLDAREAPASSWORD, and some will be transmitting
   level 1 LSPs and SNPs using the new password NEWERPASSWORD. However,
   during the second step of the transition all routers in the area will
   accept level 1 LSPs and SNPs using either password.




































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                                  Annex E
             Interaction of the Integrated IS-IS with Brouters

   A "brouter" is a device which operates an both a bridge and a router.
   One possible type of brouter acts as a router for IP traffic, and
   acts as a bridge for all other types of traffic.

   Depending upon the manner in which a brouter is implemented, and
   depending upon the network topology, there is an obscure bug which
   can result from the interaction of the Integrated IS-IS protocol, and
   brouters. This appendix gives an example of the bug, and proposes a
   simple correction to the operation of brouters to correct the
   problem.

   This annex is included for informational purposes.

E.1 The Problem

   Suppose that we have a brouter which treats IP packets as if it were
   a normal IP router, and which treats all other packets as if it is a
   bridge.

   Suppose that two routers "X" and "Y" (which implement the integrated
   IS-IS protocol), two Ethernets, and a brouter B are all connected as
   follows:


                     |                               |
                +----+---+                      +----+---+
                | Router |                      | Router |
                |   X    |                      |   Y    |
                +----+---+                      +----+---+
                     |                               |
                -----+------------+-   -+------------+----
                                  |     |
                                +-+-----+-+
                                | Brouter |
                                |    B    |
                                +---------+


   Here suppose that X and Y are running the Integrated IS-IS protocol,
   and are both level 1 routers in the same area. Thus X and Y send IS-
   IS Hello packets on the LAN. These Hello packets are received and
   forwarded by the brouter (using normal bridge functions). Similarly,
   X and Y receive each other's IS-IS LSP packets. In this way, it
   appears to the Brouter that X and Y are exchanging OSI packets, and
   so they are forwarded using normal bridge functions. It appears to X



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   and Y as if they are on the same LAN, and so they learn each others
   48-bit Ethernet addresses and exchange routing information.

   Now, suppose that X receives an IP packet, which it needs to forward
   via Y. Since X thinks that it and Y are on the same Ethernet, it just
   forwards the IP packet directly, using normal Ethernet encapsulation
   and using the 48-bit Ethernet address of Y as the destination address
   in the Ethernet header. Brouter B, when thinking as a bridge says:
   "this is an IP packet, I don't forward this as a bridge". Brouter B,
   when thinking like an IP router says: "this is an IP packet, I know
   how to forward IP packets. However, this is sent to an Ethernet
   address which is not me, thus I will ignore it". The result is that
   the IP packet does not get forwarded.

   This problem relates directly to the fact that X and Y are exchanging
   OSI packets to determine the connectivity of the path between them,
   but then are trying to send IP packets over the path. Also, there is
   a device between X and Y on the path which treats OSI and IP packets
   differently.

   Also note that this problem can also occur in more complex
   topologies, whenever a brouter is treating OSI and IP packets in a
   fundamentally different manner.

E.2 Possible Solutions

E.2.1 More Sophisticated Brouter

   One solution is that brouter B needs to be a little more
   sophisticated. In particular, it needs to use the following rules:

   - For packets which are not IP packets, act as a bridge (this is the
     same as before).

   - For IP packets sent to an Ethernet broadcast or multicast address,
     act as an IP router (this is also the same as before).

   - For IP packets sent to my own Ethernet 48-bit address(es), act as
     an IP router (this is also the same as before).

   - For IP packets sent to a single station 48-bit address which is not
     one of my addresses, act at a bridge (THIS IS NEW).

   With this change, the IP packet transmitted from X to Y is forwarded
   by the brouter, acting as a bridge. This allows the Brouter and the
   multiprotocol routers to interoperate properly.





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E.2.2 Dual Router / Brouter

   An alternate solution would be for the Brouter to route both OSI and
   IP equally. If the Brouter used the integrated IS-IS for this
   purpose, then it could be part of the same routing domain and
   interoperate like any other dual router (except for the ability to
   bridge other protocol suites).  If it used other protocols for
   routing OSI and IP, then it would need to be part of another routing
   domain, and could interoperate with integrated IS-IS routers like any
   other external router.









































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