Route Dampening

The stability of a BGP route is measured in terms of the number of times it flaps in a defined time interval. The higher the number of flaps, the less stable a route is. To prevent instability in the global routing tables due to flapping of any particular route, BGP suppresses advertisement of less stable routes until they become more stable. The method of route-flap dampening takes care of this problem. This is illustrated in Figure 11.8.

Figure 11.8: BGP route dampening occurs between AS5555 and AS6666.

In Figure 11.8, BGP is running between AS5555 and AS6666. AS5555 has routers A1 and A2, which are connected via a serial link with static routing configured. Route 10.10.1.0/24 is flapping due to flapping of the physical link between A1 and A2, as shown in these commands:

A1#show ip route | include, 00:00 S       172.16.1.0 [1/0] via 10.1.2.2, 00:00:35

The flapping of route 10.10.1.0/24 at A1 causes flapping in the routing table of B1 and also for any other AS to which AS6666 may be connected. To prevent this flapping, check BGP route flap dampening at A1 by using the command:

A1#show ip bgp dampening parameters % dampening not enabled.

This command output shows that BGP dampening is not enabled at A1. Enable route flap dampening by using the commands:

A1(config)#router bgp 5555 A1(config-router)#bgp dampening

The route starts accumulating a penalty of 1000 with each flap, as shown in Listing 11.26.

Listing 11.26 Output of show ip bgp 10.10.1.0/24 Command

A1#show ip bgp 10.10.1.0/24 BGP routing table entry for 10.10.1.0/24, version 76 Paths: (1 available, no best  path) 5555(history entry) from 10.1.2.2(10.1.2.2) Origin IGP, metric 0, external Dampinfo: penalty 1000, flapped 1 times in 0:03:23

When the penalty exceeds the suppress limit after a few flaps, the route is suppressed as shown in Listing 11.27.

Listing 11.27 Output of show ip bgp 10.10.1.0/24 Command

A1#show ip bgp 10.10.1.0/24 BGP routing table entry for 10.10.1.0/24, version 82 Paths: (1 available, no best  path) 5555(suppressed due to dampening) from 10.1.2.2(10.1.2.2) Origin IGP, metric 0, external Dampinfo: penalty 2500, flapped 3 times in 15:20:02, reuse in 00:40:30

The route is suppressed due to frequent flaps and would be re-advertised after the penalty value is less than the reuse limit. The penalty value is reduced by half in each subsequent half-life period if there are no flaps, and then can fall below the reuse limit.