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.

7-40 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

BGP Peer Group Limitations

This topic describes the limitations of BGP peer groups.

This topic describes the limitations of BGP peer groups.

Because the router builds only one update for all members of the same peer group, some restrictions apply to members of the peer group:

■ There cannot be different outbound filters, route-maps, or other means that could possibly cause different updates to be sent to two members of the same peer group. Cisco IOS software creates only one update, which is subsequently replicated to all members. Any violation of this rule could cause unexpected results.

■ External Border Gateway Protocol (EBGP) and IBGP updates are very different. EBGP updates have the AS-path attribute changed. IBGP sessions pass only the local preference attribute. The multi-exit discriminator (MED) attribute that is received from a remote AS is passed on to IBGP sessions but is removed before it is sent on an EBGP session. Therefore, you cannot assign IBGP neighbors and EBGP neighbors to the same peer group because they cannot receive a replication of the same update.

Prior to Cisco IOS Software Release 11.1(18)CC, a route reflector client could not be a member of a peer group. When the peer group leader was a route reflector client and an update was received from it, the route reflector split-horizon rules prevented the update from being sent back to the sender. Therefore, no update was generated for any members of the peer group. When using peer groups in combination with route reflectors, make sure that all routers in the AS are running Cisco IOS releases later than 11.1(18)CC, where this restriction is lifted.

Because the router sends the same update to each of the peer group members, the next-hop BGP attribute is replicated. The receivers of the information must be able to use the same next-hop IP address, requiring the receivers to be in the same IP subnet. If two receivers are on different subnets, only one of them will receive a valid next-hop attribute. The other routers will receive a next-hop IP address that is inaccessible. This restriction was also removed in Cisco IOS Release 11.1(18)CC, making BGP peer groups an ideal scalability tool.

© 2005, Cisco Systems, Inc. Optimizing BGP Scalability 7-41

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