3Com S7902E, Configuration Manual

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1-13
Besides using well-known community attributes, you can define extended community attributes by
using a community list to 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, and therefore large amounts of network and CPU resources will be
consumed.
Using route reflectors can solve this 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 routing information between
clients, and thus BGP sessions between clients need not be established.
A router that is neither a route reflector nor a client is a non-client, which has to establish BGP sessions
to the route reflector and other non-clients, as shown below.
Figure 1-13 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.
Figure 1-14 Network diagram for route reflectors
Route reflector1 Route reflector2
Client Client
Client
IBGP IBGP
IBGP
Cluster
IBGP
AS 65000
When the BGP routers in an AS are fully meshed, route reflection is unnecessary because it consumes
more bandwidth resources. You can use related commands to disable route reflection in this case.