BGP Route Reflection An alternative to full mesh IBGP
RFC 1966

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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

Bates & Chandra               Experimental                      [Page 1]
RFC 1966                  BGP Route Reflection                 June 1996

   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

Bates & Chandra               Experimental                      [Page 2]
RFC 1966                  BGP Route Reflection                 June 1996

   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.

                        +------ +        +-------+
                        |       |        |       |