Foundry Networks Switch and Router User Manual

Configuring BGP4
December 2000 19 - 73
6. Select the route map name from the Table Map field’s pulldown menu.
7. Click the Apply button to apply the changes to the device’s running-config file.
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 Layer 3 Switch suppresses
unstable routes until the route’s state changes reduce enough to meet an acceptable degree of stability. The
Foundry 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 Layer 3 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 Layer 3 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 Layer 3 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time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor, the route receives a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 Layer 3 Switch stops
using the route. Thus, by default, if a route goes down more than twice, the Layer 3 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 Layer 3
Switch. If the route's penalty falls below this value, the Layer 3 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:
BigIron(config-bgp-router)# dampening

Foundry Networks Switch and Router Specifications

General

Brand	Foundry Networks
Model	Switch and Router
Category	Switch
Language	English