A dampened route is propagated when the penalty drops below the reuse limit

© 2004 Cisco Systems, Inc. All rights reserved. BGP V3.1—7-5

A BGP router with route dampening enabled keeps track of all routes (even those that are unreachable) so that it can recall the penalties that are assigned to each route. Every time a route flap occurs, the flapping route receives 1000 penalty points. The penalty is gradually decreased through the use of a decaying algorithm. If a route flaps several times, it will be penalized (gain enough penalty points) and subsequently reach and exceed the suppress limit.

Any route that reaches the suppress limit is no longer forwarded to other neighbors until the assigned penalty is once again below the reuse limit. An exponential decay algorithm reduces penalty points that are applied to a flapping route. After the number of penalty points that are assigned to a route falls below the reuse limit, the BGP router once again advertises the route.

Copyright © 2004, Cisco Systems, Inc. Optimizing BGP Scalability 7-63

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

• Flap history is forgotten when the penalty drops below half of the "reuse limit."

• A route is never dampened for more time than the "maximum suppress limit."

• An unreachable route with a flap history is put in the "history state"—it stays in the BGP table but only to maintain the flap history.

• A penalty is applied on the individual path in the bGp table, not on the IP prefix.

A router stops tracking penalty points once they are below half of the reuse limit.

The maximum suppress limit defines the maximum duration that a route can be suppressed once it has been suppressed.

After enabling route dampening, the router never removes a route from the BGP table. A route that has been withdrawn by a BGP neighbor can still be seen in the BGP table and is marked with an "h" (history state).

A penalty is always applied to a path and not a prefix. If one of the paths is flapping, it does not mean that the destination is flapping.

7-64 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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