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Configuring Route Flap Dampening

Route flap dampening is a mechanism for minimizing instability caused by route flapping. Route flapping occurs when a link is having a problem and is constantly going up and down. Every time the link goes down, the upstream peer withdraws the routes from all its neighbors. When the link comes back up again, the peer advertises those routes globally. When the link problem appears again, the peer withdraws the routes again. This process continues until the underlying problem is fixed.

The router stores a penalty value with each route. Each time the route flaps, the router increases the penalty by 1000. If the penalty for a route reaches a configured suppress value, the router suppresses the route. That is, the router does not include the route as a forwarding entry and does not advertise the route to BGP peers.

The penalty decrements by 50 percent for each half-life interval that passes. The half-life interval resets when the route flaps and the penalty increments. The route remains suppressed until the penalty falls below the configured reuse threshold, at which point the router once again advertises the route. You can specify a max-suppress-time for route suppression; after this interval passes, the router once again advertises the route.

BGP creates a dampening parameter block for each unique set of dampening parameters—such as suppress threshold and reuse threshold—used by BGP. For example, if you have a route map that sets the dampening parameters to one set of values for some routes and to another set of values for the remaining routes, BGP uses and stores two dampening parameter blocks, one for each set.

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