BGP Route Dampening

Route dampening decreases the propagation of flapping routes. A flapping route is a route that repeatedly becomes available and then unavailable. Without route dampening, autonomous systems continually send advertisement and withdrawal messages each time the flapping route becomes available or unavailable. As the Internet grew, the number of announcements per second grew as well and caused performance problems within the routers.

Route dampening enables routers to keep a history of the flapping routes and prevent them from consuming significant network bandwidth. The routers measure how often a given route becomes available and then unavailable. When a route reaches a set threshold, that route is no longer considered valid, and is no longer propagated for a given period of time, usually about 30 minutes. If a route continues to flap even after it reaches the threshold, the time out period for that route grows in proportion to each additional flap. Once the route reaches the threshold, the route is dampened or suppressed. Suppressed routes are added back into the routing table once the penalty value decreases and falls below the reuse threshold.

Route dampening can cause connectivity to look lost to the outside world but maintained on your own network because route dampening only applies to BGP routes. Because of high load on the backbone network routers, most NSPs (MCI, Sprint, UUNet etc.) have set up route suppression.