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Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
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Chapter 35 Configuring IP Unicast Routing Configuring BGP
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:
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.
When the route reflector receives an advertised route, it takes one of these actions, depending on the
neighbor:
• A route from an external BGP speaker is advertised to all clients and nonclient peers.
• A route from a nonclient peer is advertised to all clients.
• A route from a client is advertised to all clients and nonclient peers. Hence, the clients need not be
fully meshed.
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
[autonomous-system ...] Specify the autonomous systems that belong to the
confederation and that will be treated as special EBGP peers.
Step 5 end Return to privileged EXEC mode.
Step 6 show ip bgp neighbor
show ip bgp network
Verify the configuration.
Step 7 copy running-config startup-config (Optional) Save your entries in the configuration file.