Cisco Systems 3750E Configuring Routing Domain Confederations, Configuring BGP Route Reflectors

Chapter 38 Configuring IP Unicast Routing

Configuring BGP

To delete an aggregate entry, use the no aggregate-addressaddress mask router configuration command. To return options to the default values, use the command with keywords.

Configuring Routing Domain Confederations

One way to reduce the IBGP mesh is to divide an autonomous system into multiple subautonomous systems and to group them into a single confederation that appears as a single autonomous system. Each autonomous system is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Even though the peers in different autonomous systems have EBGP sessions, they exchange routing information as if they were IBGP peers. Specifically, the next hop, MED, and local preference information is preserved. You can then use a single IGP for all of the autonomous systems.

To configure a BGP confederation, you must specify a confederation identifier that acts as the autonomous system number for the group of autonomous systems.

Beginning in privileged EXEC mode, use these commands to configure a BGP confederation:

	Command	Purpose
Step 1
	configure terminal	Enter global configuration mode.
Step 2
	router bgp autonomous-system	Enter BGP router configuration mode.
Step 3 bgp confederation identifier autonomous-system		Configure a BGP confederation identifier.
Step 4
	bgp confederation peers autonomous-system	Specify the autonomous systems that belong to the
	[autonomous-system ...]	confederation and that will be treated as special EBGP peers.
Step 5
	end	Return to privileged EXEC mode.
Step 6
	show ip bgp neighbor	Verify the configuration.
	show ip bgp network
Step 7
	copy running-config startup-config	(Optional) Save your entries in the configuration file.

Configuring BGP Route Reflectors

BGP requires that all of the IBGP speakers be fully meshed. When a router receives a route from an external neighbor, it must advertise it to all internal neighbors. To prevent a routing information loop, all IBPG speakers must be connected. The internal neighbors do not send routes learned from internal neighbors to other internal neighbors.

With route reflectors, all IBGP speakers need not be fully meshed because another method is used to pass learned routes to neighbors. When you configure an internal BGP peer to be a route reflector, it is responsible for passing IBGP learned routes to a set of IBGP neighbors. The internal peers of the route reflector are divided into two groups: client peers and nonclient peers (all the other routers in the autonomous system). A route reflector reflects routes between these two groups. The route reflector and its client peers form a cluster. The nonclient peers must be fully meshed with each other, but the client peers need not be fully meshed. The clients in the cluster do not communicate with IBGP speakers outside their cluster.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		38-61