Clearing the BGP Session

• When policies such as access lists or attributes are changed, the change takes effect immediately, and the next time that a prefix or path is advertised or received, the new policy is used. It can take a long time for the policy to be applied to all networks.

• You must trigger an update to ensure that the policy is immediately applied to all affected prefixes and paths.

• Ways to trigger an update:

- Hard reset

- Soft reset

- Route refresh

BGP can potentially handle huge volumes of routing information. When a policy configuration change occurs, the router cannot go through the huge table of BGP information and recalculate which entry is no longer valid in the local table. Nor can the router determine which route or routes, already advertised, should be withdrawn from a neighbor.

There is an obvious risk that the first configuration change will be immediately followed by a second, which would cause the whole process to start all over again. To avoid such a problem, Cisco IOS software applies changes only to those updates that are received or transmitted after the BGP policy configuration change has been performed. The new policy, enforced by the new filters, is applied only on routes that are received or sent after the change.

A network administrator who would like the policy change to be applied on all routes must trigger an update to force the router to let all routes pass through the new filter. If the filter is applied on outgoing information, the router has to resend the BGP table through the new filter. If the filter is applied on incoming information, the router needs its neighbor to resend its BGP table so that it passes through the new filters.

There are three ways to trigger an update: with a hard reset, soft reset, or route refresh.

6-78 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.

Was this article helpful?

0 0

Post a comment