3Com Switch 4800G 24-Port Configuration Manual page 377

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. If there are n routers
in an AS, the number of IBGP connections is n (n-1)/2. Therefore 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 the route reflector and non-clients, as shown below.
Figure 127 Network diagram for route reflector
IBGP
Client
Cluster
Client
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 the single point failure, as shown in the following figure. The configured
route reflectors must have the same Cluster_ID to avoid routing loops.
Route
Reflector
IBGP
IBGP
IBGP
Client
BGP Overview
Non-Client
IBGP
IBGP
Non-Client
AS 65000
377