Route Dampening

Another mechanism for controlling route instability is called route dampening. A route that appears and disappears intermittently causes BGP UPDATE and WITHDRAWN messages to be repeatedly propagated on the Internet. The tremendous amount of routing traffic generated can use up all the link's bandwidth and drive up CPU utilization of routers.

Dampening categorizes routes as either well-behaved or ill-behaved. A well-behaved route shows a high degree of stability during an extended period of time. On the other hand, an ill-behaved route experiences a high level of instability in a short period of time. Ill-behaved routes should be penalized in a way that is proportional to the expected future instability of the route. An unstable route should be suppressed (not advertised) until there is some degree of confidence that the route is stable.

---

Troubleshooting:  
Ch. 11, pp. 437-442. Route Dampening

---

The recent history of a route is used as a basis for estimating future stability. To track a route history, it is essential to track the number of times the route has flapped over a period of time. Under route dampening, each time a route flaps, it is given a penalty. Whenever the penalty reaches a predefined threshold, the route is suppressed. The route can continue to accrue penalties even after it is suppressed. The more frequently a route oscillates in a short amount of time, the faster the route will be suppressed.

Similar criteria are put in place to unsuppress a route and start readvertising it. An algorithm is implemented to decay (reduce) the penalty exponentially. The algorithm bases its configuration on a user-defined set of parameters. The following set of terms and parameters applies to the Cisco implementation:

•  Penalty—An incremented numeric value that is assigned to a route each time it flaps.

•  Half-life-time—A configurable numeric value that describes the amount of time that must elapse to reduce the penalty by one half.

•  Suppress limit—A numeric value that is compared with the penalty. If the penalty is greater than the suppress limit, the route is suppressed.

•  Reuse limit—A configurable numeric value that is compared with the penalty. If the penalty is less than the reuse limit, a suppressed route that is up will no longer be suppressed.

•  Suppressed route—A route that is not advertised, even if it is up. A route is suppressed if the penalty is more than the suppressed limit.

•  History entry—An entry used to store flap information. For the purposes of monitoring and calculating the level of oscillation of a route, it is important to store this information in the router when the route oscillates. When the route stabilizes, the history entry will become useless and will have to be flushed from the router.

Figure 9-1 illustrates the process of assessing a penalty to a route every time it flaps. The penalty is exponentially decayed according to parameters such as the half-life-time. The half-life-time parameter can be changed by the administrator to reflect the oscillation history of a route: a longer half-life might be desirable for a route that has a habit of oscillating frequently. A larger half-life-time value would cause the penalty to decay more slowly, which translates into a route being suppressed longer.

Figure 9-1  Route dampening penalty assessment.

Stability Inside the AS

The benefits of route dampening are noticed inside as well as outside an autonomous system. When BGP is redistributed (injected) into an IGP, it is important that BGP instability does not affect internal routing in such a way as to cause a meltdown inside the AS. This is where route dampening can be useful. Routes that are flapping will be dampened and prevented from being injected into the AS until they show some degree of stability. Figure 9-2 compares the effects of EBGP flapping on an IGP with and without route dampening.

Figure 9-2  Effects of EBGP flapping on an IGP.

In figure 9-2, routes R1, R2, and R3 are injected from BGP into the AS. The up and down arrows next to R2 indicate that it is flapping. The routes will be carried via IBGP and/or IGP depending on how the administrator is injecting the routes into the AS. In either case, the oscillations of R2 create major overhead for the border router and also on the interior routers. IGPs will be flooding and removing the route as long as the route is instable. With route dampening, the ill-behaved route will be suppressed (after reaching the suppress limit) and will be prevented from entering the AS.

Instabilities Outside the AS

Route dampening can prevent unstable EBGP routes from being propagated to other peers. This can save on link bandwidth usage and processing overhead within border routers. If you are a provider with multiple customers using your services, it is important not to burden your own network (and the outside world) with instabilities that go on inside a customer's network. In the case where a provider advertises a customer's network as part of an aggregate, this is not an issue. The aggregate will be stable (always advertised) even if most of its elements are not. Nonetheless, within the provider's AS, a customer's instabilities are a concern. When a customer's network cannot be aggregated (due to multihoming or addresses not being part of the provider's address space), then instabilities will be carried to the outside world.

With dampening, the provider's border router will suppress customer routes that are flapping. Suppression will take effect according to dampening rules and dampening parameters discussed earlier in this section.