rfc2796.txt

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Network Working Group                                           T. Bates
Request for Comments: 2796                                 Cisco Systems
Updates: 1966                                                 R. Chandra
Category: Standards Track                                        E. Chen
                                                        Redback Networks
                                                              April 2000


                         BGP Route Reflection -
                    An Alternative to Full Mesh IBGP

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   The Border Gateway Protocol [1] is an inter-autonomous system routing
   protocol designed for TCP/IP internets. Currently in the Internet 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.  For n BGP speakers within an AS that
   requires to maintain n*(n-1)/2 unique IBGP sessions.  This "full
   mesh" requirement clearly does not scale when there are a large
   number of IBGP speakers each exchanging a large volume of routing
   information, as is common in many of todays internet networks.





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   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". This approach allows a BGP
   speaker (known as "Route Reflector") to advertise IBGP learned routes
   to certain IBGP peers.  It represents a change in the commonly
   understood concept of IBGP, and the addition of two new optional
   transitive BGP attributes to prevent loops in routing updates.

   This document is a revision of RFC1966 [4], and it includes editorial
   changes, clarifications and corrections based on the deployment
   experience with route reflection. These revisions are summarized in
   the Appendix.

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 Transition

         It must be possible to transition 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.








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RFC 2796                  BGP Route Reflection                April 2000


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

                    Figure 1: Full Mesh IBGP

   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 advertise IBGP
   learned routes to IBGP peers, 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.





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4.  Terminology and Concepts

   We use the term "Route Reflection" to describe the operation of a BGP
   speaker advertising an IBGP learned route to another IBGP peer.  Such
   a BGP speaker is said to be a "Route Reflector" (RR), and such a
   route is said to be a reflected route.

   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, and may reflect routes
   among client peers.  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.  Figure 3 depicts a simple example outlining the
   basic RR components using the terminology noted above.

                 / - - - - - - - - - - - - -  -
                 |           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







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5. Operation

   When a RR receives a route from an IBGP peer, 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 IBGP peer

         Reflect to all the Clients.

      2) A Route from a Client peer

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

   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 possible 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
   could 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 can be
   configured with a 4-byte CLUSTER_ID so that an RR can discard routes
   from other RRs in the same cluster.




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RFC 2796                  BGP Route Reflection                April 2000


7.  Avoiding Routing Information Loops

   When a route is 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
   in reflecting a route.  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 router
   which recognizes the ORIGINATOR_ID attribute should ignore a route
   received with its ROUTER_ID as the ORIGINATOR_ID.

   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