Cisco Systems 78-11741-02 manual Configuring a Route Reflector, Confederation, IPC-317

Configuring BGP

Configuring Advanced BGP Features

In order to treat the neighbors from other autonomous systems within the confederation as special eBGP peers, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# bgp confederation peers	Specifies the autonomous systems that belong to the
as-number[as-number]	confederation.

See the “BGP Community with Route Maps Examples” section at the end of this chapter for an example configuration of several peers in a confederation.

For an alternative way to reduce the iBGP mesh, see the next section, “Configuring a Route Reflector.”

Configuring a Route Reflector

BGP requires that all iBGP speakers be fully meshed. However, this requirement does not scale well when there are many iBGP speakers. Instead of configuring a confederation, another way to reduce the iBGP mesh is to configure a route reflector.

Figure 55 illustrates a simple iBGP configuration with three iBGP speakers (Routers A, B, and C). Without route reflectors, when Router A receives a route from an external neighbor, it must advertise it to both routers B and C. Routers B and C do not readvertise the iBGP learned route to other iBGP speakers because the routers do not pass on routes learned from internal neighbors to other internal neighbors, thus preventing a routing information loop.

Figure 55 Three Fully Meshed iBGP Speakers

		Fully meshed
		autonomous
		system
		Router C
		Routes
		Routes not
	Routes
		advertised
Router A advertised
		Router A
External		Routes
BGP
speaker

Router B

S4217

With route reflectors, all iBGP speakers need not be fully meshed because there is a method to pass learned routes to neighbors. In this model, an iBGP peer is configured to be a route reflector responsible for passing iBGP learned routes to a set of iBGP neighbors. In Figure 56, Router B is configured as a route reflector. When the route reflector receives routes advertised from Router A, it advertises them to Router C, and vice versa. This scheme eliminates the need for the iBGP session between Routers A and C.

Cisco IOS IP Configuration Guide

IPC-317