Dell PowerConnect B-RX Configuration Manual page 888

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Filtering
NOTE
The BigIron RX 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 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:
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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.
Using a route map to configure route flap dampening for specific routes
Route maps enable you to fine tune route flap dampening parameters for individual routes. To
configure route flap dampening parameters using route maps, configure BGP4 address filters for
each route you want to set the dampening parameters for, then configure route map entries that
set the dampening parameters for those routes. The following sections show examples.
To configure address filters and a route map for dampening specific routes, enter commands such
as the following.
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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Each time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor,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 has a penalty value greater than
2000, the device stops using the route. Thus, 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 (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 device. If the route's 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.
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.
BigIron RX Series Configuration Guide
53-1002253-01