Cisco Systems 78-11741-02 Understanding Route Dampening Terms, Enabling Route Dampening, IPC-329

Configuring BGP

Configuring Advanced BGP Features

to advertise the status of the route to neighbors. The penalties are cumulative. When the route flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the route to network A, regardless of how many times it flaps. Thus, the route is dampened.

The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised. At half of the reuse limit, the dampening information for the route to network A is removed.

Understanding Route Dampening Terms

The following terms are used when describing route dampening:

•Flap—A route is available, then unavailable, or vice versa.

•History state—After a route flaps once, it is assigned a penalty and put into history state, meaning the router does not have the best path, based on historical information.

•Penalty—Each time a route flaps, the router configured for route dampening in another autonomous system assigns the route a penalty of 1000. Penalties are cumulative. The penalty for the route is stored in the BGP routing table until the penalty exceeds the suppress limit. At that point, the route state changes from history to damp.

•Damp state—In this state, the route has flapped so often that the router will not advertise this route to BGP neighbors.

•Suppress limit—A route is suppressed when its penalty exceeds this limit. The default value is 2000.

•Half-life—Once the route has been assigned a penalty, the penalty is decreased by half after the half-life period (which is 15 minutes by default). The process of reducing the penalty happens every 5 seconds.

•Reuse limit—As the penalty for a flapping route decreases and falls below this reuse limit, the route is unsuppressed. That is, the route is added back to the BGP table and once again used for forwarding. The default reuse limit is 750. The process of unsuppressing routes occurs at 10-second increments. Every 10 seconds, the router finds out which routes are now unsuppressed and advertises them to the world.

•Maximum suppress limit—This value is the maximum amount of time a route can be suppressed. The default value is four times the half-life.

The routes external to an autonomous system learned via iBGP are not dampened. This policy prevent the iBGP peers from having a higher penalty for routes external to the autonomous system.

Enabling Route Dampening

To enable BGP route dampening, use the following command in address family or router configuration mode:

Command	Purpose
Router(config)# bgp dampening	Enables BGP route dampening.

Cisco IOS IP Configuration Guide

IPC-329