BGP Route Dampening

Route dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when its availability alternates repeatedly. Since BGP routing tables are huge, you don’t want that many routing updates to be traveling all over the place every time a route flaps.

BGP Route Dampening Terms:

• Flap—A route whose availability alternates repeatedly
• 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.

 Configuring Dampening:

• enable
• configure terminal
• router bgp autonomous-system-number
• address-family ipv4 [unicast | multicast | vrf vrf-name]
• bgp dampening [half-life reuse suppress max-suppress-time] [route-map map-name]
• end

Checking default values:

R1#sh ip bgp dampening parameters
 dampening 15 750 2000 60 (DEFAULT)
  Half-life time      : 15 mins       Decay Time       : 2320 secs
  Max suppress penalty: 12000         Max suppress time: 60 mins
  Suppress penalty    :  2000         Reuse penalty    : 750

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.