BGP Route Flap Dampening

Route flap dampening is a BGP feature designed to minimize the propagation of flapping routes across an internetwork. A route is considered to be flapping when it repeatedly alternates between being available and being unavailable. Without route flap dampening, each transition of a route produces a withdrawal or advertisement message, which is propagated throughout the network. Route flapping can generate large numbers of messages, which can impact network bandwidth and availability.

The route flap dampening feature minimizes the flapping problem by halting route advertising and withdrawal messages for the affected route for a period of time. To support route flap dampening, the ExtremeXOS software employs a combination of two techniques. The first technique uses fixed timers to reduce the frequency of route advertisement as specified in RFC 4271. The other technique uses route flap damping algorithms. The software uses a combination of both techniques as specified in RFC 2439.

The fixed timers technique blocks all updates for the flapping route for a period defined by the internal MinRouteAdvertisementInterval (MRAI) timer (which is not configurable). For IBGP routes, this timer is set for 5 seconds. For EBGP routes, this timer is set to 30 seconds. The MRAI timer check is independent of the dampening configuration and is used to limit the frequency of route advertisements.

The route flap dampening algorithm uses configurable timers to manage route flap dampening. Suppose that the route to network 172.25.0.0 flaps. The router (in which route dampening is enabled) responds by doing the following:

Assigns route 172.25.0.0 a penalty of 1000 and moves it to a history state in which the penalty value is monitored.
Increments the penalty value by 1000 for each additional route flap.
Accumulates penalties and compares them to the suppression limit, which is set to 2000 by default.

If the suppression limit is exceeded when the MRAI timer expires, the route is not advertised to neighbors.

A route remains suppressed or dampened until one of the following events occurs:

The suppression limit is not met when the MRAI timer expires.
The penalty placed on network 172.25.0.0 is decayed below the reuse limit.
The maximum suppression timer expires.

The penalty is decayed by reducing the penalty value by one-half at the end of a configurable time period, called the half-life. Routes that flap many times may reach a maximum penalty level, or ceiling, after which no additional penalty is added. The ceiling value is not directly configurable, but the configuration parameter used in practice is the maximum route suppression time. No matter how often a route has flapped, after it stops flapping, it is advertised after the maximum route suppression time.