Many network architectures feature groups of neighbors with similar update policies. Types of groups may include these:
• Core network IBGP peers
• Distribution network RR clients
• Customers requiring only the default route, all routes for this AS, or all Internet routes Figure 11-20 demonstrates the use of peer groups within a network architecture.
Figure 11-20. The Use of Peer Groups within a Network Architecture
Full Routes Peer Group
"Defauir Peer Group
Considerable configuration effort and CPU utilization can be saved by applying peer groups in these instances. Every configuration line supplied to a peer group definition is applied to each peer group member.
The only limitation in using peer groups is that outgoing policy (route filters) must be identical for every peer group member. This is because only a single UPDATE is generated for the peer group, which is duplicated for each member of the group.
Previous limitations prevented peer groups from spanning multiple IP subnets. For example, two peer group members could not exist on separate physical interfaces or different sub-interfaces. Other limitations included providing transit between peer group members, or from being used with route reflector clients. These limitations were removed with release 12.0 of IOS, which, when necessary, modifies the next hop address to a value appropriate for each peer group member.
Unlike outgoing policy, incoming policy applied in the peer group definition may be overridden by applying a different policy in the individual neighbor configuration.
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