3Com Switch 8800 Advanced Software V5 Configuration Guide

BGP Overview 431

Community

A peer group makes peers in it enjoy the same policy, while a community makes a

group of BGP routers in several ASs enjoy the same policy. Community is a path

attribute and advertised between BGP peers, without being limited by AS.

A BGP router can modify the community attribute for a route before sending it to

other peers.

Besides using the well-known community attribute, you can define the extended

community attribute using a community list to help define a routing policy.

Route reflector

IBGP peers should be fully meshed to maintain connectivity. Suppose there are n

routers in an AS, the number of IBGP connections is n(n-1)/2. If there are many

IBGP peers, most network and CPU resources will be consumed.

Using route reflectors can solve the issue. In an AS, a router acts as a route

reflector, and other routers act as clients connecting to the route reflector. The

route reflector forwards (reflects) routing information between clients. BGP

connections between clients need not be established.

The router neither a route reflector nor a client is a non-client, which has to

establish connections to all the route reflector and non-clients, as shown below.

Figure 143 Network diagram for route reflector

The route reflector and clients form a cluster. In some cases, you can configure

more than one route reflector in a cluster to improve network reliability and

prevent single point failure, as shown in the following figure. The configured route

reflectors must have the same Cluster_ID to avoid routing loops.

Client

Client Client

Route

Reflector

Non-Client

Cluster

AS 65000

IBGP