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Configuring IP Unicast Routing
Configuring BGP
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.
Usually a cluster of clients have a single route reflector, and the cluster is identified by the route reflector router ID. To
increase redundancy and to avoid a single point of failure, a cluster might have more than one route reflector. In this case,
all route reflectors in the cluster must be configured with the same 4-byte cluster ID so that a route reflector can
recognize updates from route reflectors in the same cluster. All the route reflectors serving a cluster should be fully
meshed and should have identical sets of client and nonclient peers.
BEFORE YOU BEGIN
Enable BGP routing as described in the Enabling BGP Routing, page 850.
DETAILED STEPS
EXAMPLE
In the following router configuration mode example, the local router is a route reflector. It passes learned IBGP routes to
the neighbor at 172.16.70.24.
Command Purpose
1. configure terminal Enter global configuration mode.
2. router bgp autonomous-system Enter BGP router configuration mode.
3. neighbor ip-address | peer-group-name
route-reflector-client
Configure the local router as a BGP route reflector and
the specified neighbor as a client.
4. bgp cluster-id cluster-id (Optional) Configure the cluster ID if the cluster has
more than one route reflector.
5. no bgp client-to-client reflection (Optional) Disable client-to-client route reflection. By
default, the routes from a route reflector client are
reflected to other clients. However, if the clients are fully
meshed, the route reflector does not need to reflect
routes to clients.
6. end Return to privileged EXEC mode.
7. show ip bgp Verify the configuration. Display the originator ID and the
cluster-list attributes.
8. copy running-config startup-config (Optional) Save your entries in the configuration file.
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