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While reading this tutorial, remember that BGP is a routing protocol. That means BGP learns and selects the best route to a given destination. Anything that makes a route less preferred causes the router to stop adding the route to the IP routing table and to stop advertising the route to neighbors.

What is a Route Flap?

BGP peers exchange routes and send updates not faster than every 90 seconds by default. When a route is repeatedly advertised and withdrawn, it is considered to be 'flapping'. Flapping routes cause instability in the Internet routing table and Cisco routers running BGP contain an optional mechanism designed to dampen the destabilizing effect of flapping routes. When a Cisco router running BGP detects a flapping route it automatically dampens that route. The route dampening prevents routers from thrashing while trying to re-calculate a large number of route updates. The overall effect is to produce a more stable routing table. BGP routes can remain in the routing table for months.

The term route flap is used when a previously advertised route is withdrawn and then readvertised. Cisco IOS later than 11.0 has a route dampening function built into it. If you are running BGP version 4, the BGP process assigns a penalty of 1000 to the route each time it flaps. When the penalty value exceeds the first of two limits, the route is moved into the 'historical' list of routes, dampened, and then is no longer accepted from other peers or announced to any peers. After the first limit has been exceeded, the timer which tracks the period for which the route is to be dampened is doubled for each flap.

The suppression half-life is 15 minutes. The maximum suppress limit is four times the half-life; thus, one hour is the default. The suppression penalty decays at half the half life (7.5 minutes). So:

  1. First flap, penalty 1000 assigned, route placed in 'historical' category and becomes less preferred.
  2. Second flap, route has met the suppression limit of 2000 (a Cisco default). The route is dampened and no longer advertised to neighbors or accepted from neighbors.
  3. If route does not flap any further the penalty is decayed. The decay process begins 7.5 minutes after the route stabilized and decays exponentially every 5 seconds thereafter.
  4. Once the suppression penalty decays below 750 (the default value for the reuse threshold), the route is removed from dampened state and reused. The router parses the historical routes list every 10 seconds for reusable routes.

Checking for Dampened BGP Paths

You can check for dampened paths by issuing the following command at the command prompt:

router# show ip bgp dampened-paths

This works on Cisco, Juniper, Avici and HP routers.


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