Brocade Communications Systems FastIron SX 1600 Configuration Manual Download

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Configuring route flap dampening

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

Route flap dampening helps reduce the impact of route flap by changing the way a BGP4 device responds to route state changes. When
route flap dampening is configured, the device suppresses unstable routes until the number of route state changes drops enough to
meet an acceptable degree of stability. The Brocade 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 device 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 device stops using
that route and stops advertising it to other devices. The mechanism also allows route penalties to reduce over time if route stability
improves.

The route flap dampening mechanism uses the following parameters:

•

Suppression threshold - Specifies the penalty value at which the device 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unreachable or is withdrawn by a BGP4 UPDATE from a neighbor, the route receives a penalty of 1000.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 penalty is greater than 2000, the device stops using the route. By default, if a route goes down more than twice, the
device stops using the route. You can set the suppression threshold to a value from 1 through 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 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 device. If the route penalty
falls below this value, the device 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 through 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 through 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

Route flap dampening reduces the amount of route state changes propagated by BGP4 due to unstable routes. This in turn reduces
processing requirements.

Brocade Communications Systems FastIron SX 1600, Configuration Manual

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