Dampening Saga

Terms used describing route dampening:

• Flap—A route is available, then unavailable, or vice versa; WITHDRAW/UPDATE
• 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.

Note the following:

MaxPenalty = ReuseLimit * 2 ^ (MaxSupressTime / HalfLife) 

    Penalty :1000
    Suppress Limit: 10000
    Reuse Limit: 1500
    Half-Life: 30 Minutes
    Maximum Suppress-Limit: 60 Minutes

bash:$> echo $(( 1500*2 ** (60/30) ))
6000
bash:$>  

route flap causes a penalty of 1000. When the penalty reaches 10,000, the prefix gets dampened. However, the maximum penalty that can be assigned is 6000. This means we will never incur a penalty significant enough to dampen the prefix.

• Each time an eBGP route flaps, it gets 1000 penalty points (this cannot be configured or changed). 
• iBGP routes are not dampened.
• A route also 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 (marked with h in the BGP table).
• A penalty is applied on the individual path in the BGP table, not on the IP prefix.
• clear ip bgp * is regarded as a flap, which could cause that path to be suppressed.
• clear ip bgp * [soft] in is not regarded as a flap to neighbors.