Configuring BGP4

8.Select the Save link at the bottom of the dialog. Select Yes when prompted to save the configuration change to the startup-config file on the device’s flash memory.

Configuring Route Flap Dampening

A “route flap” is the change in a route’s state, from up to down or down to up. When a route’s state changes, the state change causes changes in the route tables of the routers that support the route. Frequent changes in a route’s state can cause Internet instability and add processing overhead to the routers that support the route.

Route flap dampening is a mechanism that reduces the impact of route flap by changing a BGP4 router’s response to route state changes. When route flap dampening is configured, the routing switch suppresses unstable routes until the route’s state changes reduce enough to meet an acceptable degree of stability. The HP implementation of route flap dampening is based on RFC 2439.

Route flap dampening is disabled by default. You can enable the feature globally or on an individual route basis using route maps.

NOTE: The routing switch applies route flap dampening only to routes learned from EBGP neighbors.

The route flap dampening mechanism is based on penalties. When a route exceeds a configured penalty value, the routing switch stops using that route and also stops advertising it to other routers. The mechanism also allows a route’s penalties to reduce over time if the route’s stability improves. The route flap dampening mechanism uses the following parameters:

•Suppression threshold – Specifies the penalty value at which the routing switch stops using the route. Each time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor, the route receives a penalty of 1000. By default, when a route has a penalty value greater than 2000, the routing switch stops using the route. Thus, by default, if a route goes down more than twice, the routing switch stops using the route. You can set the suppression threshold to a value from 1 – 20000. The default is 2000.

•Half-life – Once a route has been assigned a penalty, the penalty decreases exponentially and decreases by half after the half-life period. The default half-life period is 15 minutes. The software reduces route penalties every five seconds. For example, if a route has a penalty of 2000 and does not receive any more penalties (it does not go down again) during the half-life, the penalty is reduced to 1000 after the half-life expires. You can configure the half-life to be from 1 - 45 minutes. The default is 15 minutes.

•Reuse threshold – Specifies the minimum penalty a route can have and still be suppressed by the routing switch. If the route's penalty falls below this value, the routing switch un-suppresses the route and can use it again. The software evaluates the dampened routes every ten seconds and un-suppresses the routes that have penalties below the reuse threshold. You can set the reuse threshold to a value from 1 - 20000. The default is 750.

•Maximum suppression time – Specifies the maximum number of minutes a route can be suppressed regardless of how unstable the route has been before this time. You can set the parameter to a value from 1 – 20000 minutes. The default is four times the half-life. When the half-life value is set to its default (15 minutes), the maximum suppression time defaults to 60 minutes.

You can configure route flap dampening globally or for individual routes using route maps. If you configure route flap dampening parameters globally and also use route maps, the settings in the route maps override the global values.

Globally Configuring Route Flap Dampening

To configure route flap dampening globally, use either of the following methods.

USING THE CLI

To enable route flap dampening using the default values, enter the following command:

HP9300(config-bgp-router)# dampening

Syntax: dampening [<half-life> <reuse> <suppress> <max-suppress-time>]

10 - 69