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.
AbstractThe Border Gateway Protocol  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.
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 , and it includes editorial changes, clarifications and corrections based on the deployment experience with route reflection. These revisions are summarized in the Appendix. 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.
+-------+ +-------+ | | 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.
are often fully meshed for the purpose of optimal intra-POP routing, and the intra-POP IGP metrics are configured to be better than the inter-POP IGP metrics. 5].  Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC 1771, March 1995.  Haskin, D., "A BGP/IDRP Route Server alternative to a full mesh routing", RFC 1863, October 1995.  Traina, P., "Limited Autonomous System Confederations for BGP", RFC 1965, June 1996.  Bates, T. and R. Chandra, "BGP Route Reflection An alternative to full mesh IBGP", RFC 1966, June 1996.  Heffernan, A., "Protection of BGP Sessions via the TCP MD5 Signature Option", RFC 2385, August 1998.
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