Peer Groups as a BGP Performance Tool

Cisco.com

• Cisco IOS software builds individual BGP updates for each BGP neighbor.

- The CPU load imposed by the BGP process is proportional to the number of BGP neighbors.

• A single BGP update is built for all members of a BGP peer group.

- The CPU load does not increase linearly with the increased number of neighbors.

- Hint: Use peer groups wherever possible to reduce the CPU load of the BGP process.

By default, Cisco IOS software builds BGP updates for each neighbor individually. Building BGP updates involves a number of router-CPU-consuming tasks, including scanning the BGP table and applying a variety of outgoing filtering mechanisms (filter-lists, route-maps, and prefix-lists). These tasks imply that when a router is configured with a large number of neighbors, the CPU load grows proportionally.

However, with the use of peer groups, some of the router CPU utilization that is imposed by BGP update generation is significantly reduced, because the use of peer groups allows the router to run the BGP update (including all outgoing filter processing) only once for the entire peer group. The router, after it has finished building the BGP update, sends it to each member of the peer group. The actual TCP transmission still has to be done on a per-neighbor basis because of the connection-oriented characteristics of BGP sessions.

So, router CPU load does increase when there are more neighbors of a router, because of increased TCP workload, but the use of peer groups can significantly reduce the increase. Therefore, network administrators should use peer groups whenever possible to reduce the CPU load.

Note BGP peer groups are the fundamental BGP scalability tool and should be used in all environments where a router has a large number of BGP neighbors.

Copyright © 2004, Cisco Systems, Inc. Optimizing BGP Scalability 7-43

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