3Com 10014299 manual Schematic diagram of route dampening

464CHAPTER 30: CONFIGURING BGP

Configuring Route Route instability is frequently indicated when a route disappears that used to exist Dampening in the routing table. This route may reappear and disappear frequently, which is

called routing flapping. When there is route flapping, the UPDATE and WITHDRAWN messages are broadcast repeatedly over the network, occupying bandwidth and processing time of the routers. The administrator should take action to prevent route flapping. Route dampening is a technology to control routing flapping.

There are two types of routes, stable routes and unstable routes. Stable routes remain in the route table continuously, while unstable routes should be suppressed by route dampening. The unstable route is penalized by not allowing it to advertise when its penalty level reaches a threshold. The penalty is exponentially decreased as time goes by. Once it is lower than a certain threshold, the route is unsuppressed and is advertised again, as shown in the following diagram.

Figure 153 Schematic diagram of route dampening

Penalty

Suppression threshold

Threshold to reuse

Tim e

Configure the following parameters to adjust the performance of route dampening:

■Penalty: Increases upon each route flap, decays as time goes by.

■Reachable-half-life:Time duration before they reachable route penalty is reduced to half.

■Unreachable-half-time:Time duration before the unreachable route penalty is reduced to half.

■Ceiling-max-suppress:The maximum value of the penalty.

■Suppress-limit:The route advertisement is suppressed when the penalty reaches this threshold.

■Reuse-limit:The route advertisement is unsuppressed when the penalty is lower than this value.

1Configure route dampening.

Perform the following configurations in BGP view.

The parameters are mutually dependent. To configure any parameter, all other parameters should also be specified.