H3C Technologies H3C S7500E Series Switches - Manual (Page 216)

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If an option is configured for both a peer and its peer group, the last configuration takes effect.
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 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 6-13 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.