Flapping routes are bad news in BGP. flapping routes are the routes that go up and down in very short time. they are bad news cause they require changes in routing table. routing table that consists of 100 thousand routes cause high load on router. a flapping route will: 1- change the table by being removed and added. 2- change other routes that depend on that route for transition (recursive lookup).
To minimize flapping routes, two separate features can be used. First is the route summary. Second is the route dampening.
Route dampening is to suppress a prefix based on the number of flaps. each flap will have a penalty value (cost). once the route flaps, the route will be added to “history” where it will be tracked. If the suppress limited reached BGP will will suppress the route and mark it as damped. Then every 5 seconds the penalty value will be decreased by exponentially. The decrease value depends on one single parameter which is half-life. half-life = the amount of time required to make the penalty value half of the current state. The penalty for flapped route is 1000 per flap, while penalty for attribute change is 500.
The equation for the decay is
P(t) = P(0) / 2^(t/half-life); Where p(t) = reuse limit, P(0) = suppress limit.
the command syntax to apply this in BGP is
router bgp xxxxx
bgp dampening [half-life reuse suppress max-suppress-time] [route-map map-name]
The default values are half-life = 15 mins, reuse = 750, suppress = 2000, max-suppress-time=60 mins.
The route map can be used to set these values as well. The mentioned equation is important to calculate the half-time values, cause they determine how fast a flapping route can be released from the damped stated.