9.2
Route Filtering and Manipulation Process (Policies)
9.2.8 Peer group
A BGP peer group is a group of BGP neighbors that share the same update policies. Instead of defining the same policies for each individual neighbor, you define a peer group and then assign policies to the peer group itself.

Not only do peer groups save the operator from repetitive configuration of each BGP peer, they also save the BGP router itself from the effort of parsing the policies sequentially for each neighbor. With peer groups, the router formulates the UPDATE once, based on the policies of the peer group, and then floods the same UPDATE to all the neighbors that fall within the group.

In the Figure, RTA has three internal peers with which it has the same internal policies', and three external peers with which it has the same policies. RTA's configuration includes two sets of peer groups, one for inside the AS and one for outside the AS. Each peer group contains the set of policies that RTA has toward its peers. These policies could be a set of IP prefix filters or AS_path filters and possible attribute manipulation. After the peer groups have been defined, these policies are applied to the neighbors that make up the peer group.

Because of the route update optimization that peer groups offer, some restrictions need to be followed for peer groups to work correctly with external BGP peers. If the following guidelines are not followed, loss of routing information could occur.

When the peer group consists of external neighbors (EBGP), the following restrictions must apply:

  • The hub router (such as RTA in the figure) cannot be a transit router for the external ASs. In other words, updates from one EBGP neighbor in the peer group should not be passed to other EBGP neighbors in the same peer group.

Peer Group Exceptions:

Exceptions occur when some neighbors inside a peer group have slightly different policies from other neighbors. Additional policies can be added to the neighbor to complement the set of policies that fall within the peer group. Assume that RTA requires an additional set of filters to be set toward its peer RTB. RTA can apply the extra filters toward RTB while still keeping RTB within the external peer group.