





Network Working Group                                           T. Bates
Request for Comments: 1966                                 cisco Systems
Category: Experimental                                        R. Chandra
                                                           cisco Systems
                                                               June 1996


                          BGP Route Reflection
                    An alternative to full mesh IBGP

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Abstract

   The Border Gateway Protocol [1] is an inter-autonomous system routing
   protocol designed for TCP/IP internets. BGP deployments are
   configured such that that all BGP speakers within a single AS must be
   fully meshed so that any external routing information must be re-
   distributed to all other routers within that AS. This represents a
   serious scaling problem that has been well documented with several
   alternatives proposed [2,3].

   This document describes the use and design of a method known as
   "Route Reflection" to alleviate the the need for "full mesh" IBGP.

1.  Introduction

   Currently in the Internet, BGP deployments are configured such that
   that all BGP speakers within a single AS must be fully meshed and any
   external routing information must be re-distributed to all other
   routers within that AS. This "full mesh" requirement clearly does not
   scale when there are a large number of IBGP speakers as is common in
   many of todays internet networks.

   For n BGP speakers within an AS you must maintain n*(n-1)/2 unique
   IBGP sessions. With finite resources in both bandwidth and router CPU
   this clearly does not scale.

   This scaling problem has been well documented and a number of
   proposals have been made to alleviate this [2,3]. This document
   represents another alternative in alleviating the need for a "full
   mesh" and is known as "Route Reflection". It represents a change in
   the commonly understood concept of IBGP and the addition of two new



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   optional transitive BGP attributes.

2.  Design Criteria

   Route Reflection was designed to satisfy the following criteria.

           o Simplicity

             Any alternative must be both simple to configure as well
             as understand.

           o Easy Migration

             It must be possible to migrate from a full mesh
             configuration without the need to change either topology
             or AS. This is an unfortunate management overhead of the
             technique proposed in [3].

           o Compatibility

             It must be possible for non compliant IBGP peers
             to continue be part of the original AS or domain
             without any loss of BGP routing information.

   These criteria were motivated by operational experiences of a very
   large and topology rich network with many external connections.

3.  Route Reflection

   The basic idea of Route Reflection is very simple. Let us consider
   the simple example depicted in Figure 1 below.

                        +------ +        +-------+
                        |       |  IBGP  |       |
                        | RTR-A |--------| RTR-B |
                        |       |        |       |
                        +-------+        +-------+
                              \             /
                          IBGP \   ASX     / IBGP
                                \         /
                                 +-------+
                                 |       |
                                 | RTR-C |
                                 |       |
                                 +-------+

                         Figure 1: Full Mesh IBGP




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   In ASX there are three IBGP speakers (routers RTR-A, RTR-B and RTR-
   C).  With the existing BGP model, if RTR-A receives an external route
   and it is selected as the best path it must advertise the external
   route to both RTR-B and RTR-C. RTR-B and RTR-C (as IBGP speakers)
   will not re-advertise these IBGP learned routes to other IBGP
   speakers.

   If this rule is relaxed and RTR-C is allowed to reflect IBGP learned
   routes, then it could re-advertise (or reflect) the IBGP routes
   learned from RTR-A to RTR-B and vice versa. This would eliminate the
   need for the IBGP session between RTR-A and RTR-B as shown in Figure
   2 below.

                        +------ +        +-------+
                        |       |        |       |
                        | RTR-A |        | RTR-B |
                        |       |        |       |
                        +-------+        +-------+
                             \             /
                         IBGP \   ASX     / IBGP
                               \         /
                                 +-------+
                                 |       |
                                 | RTR-C |
                                 |       |
                                 +-------+

                      Figure 2: Route Reflection IBGP

   The Route Reflection scheme is based upon this basic principle.

4.  Terminology and Concepts

   We use the term "Route Reflector" (RR) to represent an IBGP speaker
   that participates in the reflection.  The internal peers of a RR are
   divided into two groups:

           1) Client Peers

           2) Non-Client Peers

   A RR reflects routes between these groups.  A RR along with its
   client peers form a Cluster. The Non-Client peer must be fully meshed
   but the Client peers need not be fully meshed. The Client peers
   should not peer with internal speakers outside of their cluster.
   Figure 3 depicts a simple example outlining the basic RR components
   using the terminology noted above.




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                      / - - - - - - - - - - - - -  -\
                      |           Cluster           |
                        +-------+        +-------+
                      | |       |        |       |  |
                        | RTR-A |        | RTR-B |
                      | |Client |        |Client |  |
                        +-------+        +-------+
                      |      \             /        |
                         IBGP \           / IBGP
                      |        \         /          |
                                +-------+
                      |         |       |           |
                                | RTR-C |
                      |         |  RR   |           |
                                +-------+
                      |           /   \             |
                      \ - - - - -/- - -\- - - - - - /
                          IBGP  /       \  IBGP
                       +-------+         +-------+
                       | RTR-D |  IBGP   | RTR-E |
                       |  Non- |---------|  Non- |
                       |Client |         |Client |
                       +-------+         +-------+

                          Figure 3: RR Components

5. Operation

   When a route is received by a RR, it selects the best path based on
   its path selection rule. After the best path is selected, it must do
   the following depending on the type of the peer it is receiving the
   best path from:

           1) A Route from a Non-Client peer

              Reflect to all other Clients.

           2) A Route from a Client peer

              Reflect to all the Non-Client peers and also to the
              Client peers other than the originator. (Hence the
              Client peers are not required to be fully meshed).

            3) Route from an EBGP peer

               Send to all the Client and Non-Client Peers.





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   An Autonomous System could have many RRs. A RR treats other RRs just
   like any other internal BGP speakers. A RR could be configured to
   have other RRs in a Client group or Non-client group.

   In a simple configuration the backbone could be divided into many
   clusters.  Each RR would be configured with other RRs as Non-Client
   peers (thus all the RRs will be fully meshed.). The Clients will be
   configured to maintain IBGP session only with the RR in their
   cluster.  Due to route reflection, all the IBGP speakers will receive
   reflected routing information.

   It is normal in a Autonomous System to have BGP speakers that do not
   understand the concept of Route-Reflectors (let us call them
   conventional BGP speakers). The Route-Reflector Scheme allows such
   conventional BGP speakers to co-exist. Conventional BGP speakers ould
   be either members of a Non-Client group or a Client group. This
   allows for an easy and gradual migration from the current IBGP model
   to the Route Reflection model. One could start creating clusters by
   configuring a single router as the designated RR and configuring
   other RRs and their clients as normal IBGP peers. Additional clusters
   can be created gradually.

6.  Redundant RRs

   Usually a cluster of clients will have a single RR. In that case, the
   cluster will be identified by the ROUTER_ID of the RR. However, this
   represents a single point of failure so to make it possible to have
   multiple RRs in the same cluster, all RRs in the same cluster must be
   configured with a 4-byte CLUSTER_ID so that an RR can discern routes
   from other RRs in the same cluster.

7.  Avoiding Routing Information Loops

   As IBGP learned routes are reflected, it is possible through mis-
   configuration to form route re-distribution loops. The Route
   Reflection method defines the following attributes to detect and
   avoid routing information loops.

   ORIGINATOR_ID

   ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type
   code 9.  This attribute is 4 bytes long and it will be created by a
   RR. This attribute will carry the ROUTER_ID of the originator of the
   route in the local AS. A BGP speaker should not create an
   ORIGINATOR_ID attribute if one already exists.  A route reflector
   must never send routing information back to the router specified in
   ORIGINATOR_ID.




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   CLUSTER_LIST

   Cluster-list is a new optional, non-transitive BGP attribute of Type
   code 10. It is a sequence of CLUSTER_ID values representing the
   reflection path that the route has passed. It is encoded as follows:


              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Attr. Flags  |Attr. Type Code|   Length      | value ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where Length is the number of octets.

   When a RR reflects a route from its Clients to a Non-Client peer, it
   must append the local CLUSTER_ID to the CLUSTER_LIST. If the
   CLUSTER_LIST is empty, it must create a new one. Using this attribute
   an RR can identify if the routing information is looped back to the
   same cluster due to mis-configuration. If the local CLUSTER_ID is
   found in the cluster-list, the advertisement will be ignored.

8.  Implementation and Configuration Considerations

   Care should be taken to make sure that none of the BGP path
   attributes defined above can be modified through configuration when
   exchanging internal routing information between RRs and Clients and
   Non-Clients. This could result is looping of routes.

   In some implementations, modification of the BGP path attribute,
   NEXT_HOP is possible. For example, there could be a need for a RR to
   modify NEXT_HOP for EBGP learned routes sent to its internal peers.
   However, it must not be possible for an RR to set on reflected IBGP
   routes as this breaks the basic principle of Route Reflection and
   will result in potential black holeing of traffic.

   An RR should not modify any AS-PATH attributes (i.e. LOCAL_PREF, MED,
   DPA)that could change consistent route selection. This could result
   in potential loops.

   The BGP protocol provides no way for a Client to identify itself
   dynamically as a Client to an RR configured BGP speaker and the
   simplest way to achieve this is by manual configuration.

9.  Security Considerations

   Security issues are not discussed in this memo.





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10. Acknowledgments

   The authors would like to thank Dennis Ferguson, Enke Chen, John
   Scudder, Paul Traina and Tony Li for the many discussions resulting
   in this work. This idea was developed from an earlier discussion
   between Tony Li and Dimitri Haskin.

11. References

   [1]  Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
        RFC 1771, March 1995.

   [2]  Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh
        routing", RFC 1863, October 1995.

   [3]  Traina, P., "Limited Autonomous System Confederations for BGP",
        RFC 1965, June 1996.

12. Authors' Addresses

   Tony Bates
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134

   Phone: +1 408 527 2470
   EMail: tbates@cisco.com


   Ravishanker Chandrasekeran
   (Ravi Chandra)
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: rchandra@cisco.com















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