Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 277)

Brocade Virtual ADX Switch and Router Guide 263
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Filtering
8
BGP4 policy processing order
The order of application of policies when processing inbound and outbound route advertisements
on the device is:
1. lp prefix-list
2. Filter-list (using As-path access-list)
3. Distribute list (using IP ACL - ipv4 unicast only)
4. Route-map
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Each time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor,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 – 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 – 20000. The default is 750.