Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Instrukcja Użytkownika Pobierz pdf (Strona 89)

Configuring and Customizing BGP

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Configuring EBGP Route Flap Dampening

The frequent change of network reachability information that can be caused by an

unstable route is commonly referred to as route flap. Route flap dampening is a

technique for suppressing information about unstable routes.

BayRS supports dampening of unstable EBGP routes. The route flap dampening

mechanism performs the following functions:

• Determines the stability of an EBGP route

• Suppresses the use and advertisement of unstable EBGP routes

• Unsuppresses a route that has regained stability

To determine the stability of a route in the IP routing table, BGP maintains a

penalty value for the route based on its recent history. Each time the route flaps

(that is, each time it is withdrawn from the routing table), BGP increments this

penalty value. During the period of time that the route does not flap, BGP

decrements the penalty value. In this way, as the instability of the route increases,

the penalty value rises. As the route becomes more stable, the penalty value falls.

To monitor BGP for route flaps, you set the route dampening parameter on a BGP

accept policy that matches the route. BGP creates a penalty value for the route and

sets the value to 0. BGP increments the penalty each time the route flaps.

BGP provides a route flap dampening template that allows you to control the way

BGP evaluates a route’s penalty and how, based on the penalty, it decides to

suppress or unsuppress the route. The template consists of the following

parameters:

• Cutoff threshold. If the penalty value rises above the value that you specify as

the cutoff threshold, BGP suppresses the route.

• Reuse threshold. If the penalty value falls below the value that you specify as

the reuse threshold, BGP unsuppresses the route.

• Reachable decay. If the route is still reachable after the period of time that you

specify, BGP reduces the penalty value by half. (BGP uses a half-life decay

algorithm to decrement the penalty value.)

• Unreachable decay. If the route is still unreachable after the period of time

that you specify, BGP reduces the penalty value by half. (BGP uses a half-life

decay algorithm to decrement the penalty value.)

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Avaya Configuring IP Exterior Gateway Protocols (BGP and EGP) Instrukcja Użytkownika Strona 89