H3C S5600 SERIES Operation Manual page 338

Before sending a route with the COMMUNITY attribute to its peers, a BGP router can change the
original COMMUNITY attribute of the route.
Besides the well-known COMMUNITY attributes, you can also use the COMMUNITY attributes list to
customize extended COMMUNITY attributes, so as to control the routing policy with more flexibility.
Router reflector
To ensure the connectivity among the IBGP peers in an AS, you need to make the IBGP peers fully
connected. For an AS with the number of the routers in it being n, you need to establish at least
n*(n-1)/2 IBGP connections to make them fully connected. This requires large amount of network
resources and CPU time if large amount of IBGP peers exist in the AS.
You can decrease the use of network resources and CPU time through route reflection in this case. That
is, use a router as a router reflector (RR) and establish IBGP connections between the RR and other
routers known as clients. Routing information exchanged between the clients is passed/reflected by the
RR. This eliminates the need to establish IBGP connections among the clients.
Note that a BGP router which is neither the RR nor a client is called a non-client. Non-clients and the RR
must be fully connected, as shown in Figure 5-11.
Figure 5-11 Diagram for the route reflector
An RR and all its clients form a cluster. To ensure network reliability and avoid single-point failure, you
can configure more than one RR in a cluster. In this case, make sure all the RRs in the cluster are
configured with the same cluster ID to avoid routing loops. Figure shows a cluster containing two RRs.
5-11