Route dampning in BGP

Route dampening is the feature that reduces propagation of flapping routes in the network. Route flapping occurs when IP routes are removed and put back in a routing table. This can be because of physical layer failure, routing protocol failure, or router node failure, and so on. When these flaps are announced through BGP to the Internet, all of Internet routers running BGP are affected, as they have to remove and install such flapping routes. In an unstable internal network where IP routes continuously flaps, this instability is propagated through BGP throughout the Internet. Route dampening is the feature that minimizes this instability by assigning a penalty to such flapping routes. When the penalty reaches a predefined limit (suppress limit), that route is removed from the routing table and is not advertised to Internet. When the route stops flapping, the penalty decreases exponentially. When the penalty is reduced to a predefined limit (reuse limit), that route is installed again and propagated through BGP. Some of the rules and definitions regarding route dampening are as follows: 

● Cisco IOS Software application— Route dampening applies to EBGP neighbors only.

● Flap penalty:- Each flap receives a penalty of 1000. A penalty is assigned only when routes are withdrawn and not when they are re-advertised.

● Suppress limit— A route is suppressed and removed from the routing table if the penalty exceeds this limit. The default suppress limit is 2000.

● Half-life time— Every 5 sec, a penalty is exponentially reduced such that in half-life the penalty will be reduced to half of its value. Default Half-Life Time is 15 minutes.

● Reuse limit— With exponential reduction of penalty, a penalty will reach its reuse limit at which the route will no longer be suppressed and will be installed and advertised to other BGP speaker. The Reuse-limit default is 750. When penalty is half of Reuse-limit, the dampening information will be purged.

● History state— When flap (withdrawal) occurs, a route is assigned a penalty of 1000. In this state, BGP does not have the route because it is withdrawn, but BGP maintain the information about the route in history state to keep track of dampening.

● Damp state— With repeated flaps, where the penalty exceeds the suppress limit, the route is removed from routing table and is not advertised to any BGP speaker.

● Maximum duration of dampening— Default is 4 times of half-life time (15 minutes). A route can only be dampened for 1 hour in default settings.