Extreme Networks ExtremeWare XOS Guide Manual page 406

Exterior Gateway Routing Protocols
Minimizing the Route Flap
The route flap dampening feature minimizes the flapping problem as follows. Suppose that the route to
network 172.25.0.0 flaps. The router (in which route dampening is enabled) assigns network 172.25.0.0 a
penalty of 1000 and moves it to a "history" state in which the penalty value is monitored. The router
continues to advertise the status of the route to neighbors. The penalties are cumulative. When the route
flaps so often that the penalty exceeds a configurable suppress limit, the router stops advertising the
route to network 172.25.0.0, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network 172.25.0.0 is decayed until the reuse limit is reached, when the route is
once again advertised. At half of the reuse limit, the dampening information for the route to network
172.25.0.0 is removed.
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, once it stops flapping, it will again be advertised after the maximum route
suppression time.
Configuring Route Flap Dampening
Using a route map, you enable BGP route flap dampening per BGP peer session, for a BGP peer group,
or for a set of routes.
To enable route flap dampening over BGP peer sessions, use the following command:
configure bgp neighbor [all | <remoteaddr>] {address-family [ipv4-unicast | ipv4-
multicast]} dampening {{half-life <half-life-minutes> {reuse-limit <reuse-limit-
number> suppress-limit <suppress-limit-number> max-suppress <max-suppress-minutes>} |
policy-filter [<policy-name> | none]}
To enable route flap dampening for a BGP peer group, use the following command:
configure bgp peer-group <peer-group-name> {address-family [ipv4-unicast | ipv4-
multicast]} dampening {{half-life <half-life-minutes> {reuse-limit <reuse-limit-
number> supress-limit <suppress-limit-number> max-suppress <max-suppress-minutes>}} |
policy-filter [<policy-name> | none]}
You can supply the dampening parameters directly through the command line interface (CLI)
command, or use the command to associate a policy that contains the desired parameters.
Disabling Route Flap Dampening
To disable route flap dampening for a BGP neighbor (disabling the dampening also deletes all the
configured dampening parameters), use the following command:
configure bgp neighbor [<remoteaddr> | all] {address-family [ipv4-unicast | ipv4-
multicast]} no-dampening
To disable route flap dampening for a BGP peer group, use the following command:
configure bgp peer-group <peer-group-name> no-dampening
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ExtremeWare XOS 11.1 Concepts Guide