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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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
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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
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