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Configuring BGP4
December 2005
© Foundry Networks, Inc.
21 - 59
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time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor, the route receives a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 enable route flap dampening using the default values, enter the following command:
FESX424 Router(config-bgp-router)# dampening
Syntax:
dampening [<half-life> <reuse> <suppress> <max-suppress-time>]
The <half-life> parameter specifies the number of minutes after which the route’s penalty becomes half its value.
The route penalty allows routes that have remained stable for a while despite earlier instability to eventually
become eligible for use again. The decay rate of the penalty is proportional to the value of the penalty. After the
half-life expires, the penalty decays to half its value. Thus, a dampened route that is no longer unstable can
eventually become eligible for use again. You can configure the half-life to be from 1 - 45 minutes. The default is
15 minutes.
The <reuse> parameter specifies how low a route’s penalty must become before the route becomes eligible for
use again after being suppressed. You can set the reuse threshold to a value from 1 – 20000. The default is 750
(0.75, or three-fourths, of the penalty assessed for a one “flap”).
The <suppress> parameter specifies how high a route’s penalty can become before the Layer 3 Switch
suppresses the route. You can set the suppression threshold to a value from 1 – 20000. The default is 2000 (two
“flaps”).
The <max-suppress-time> parameter specifies the maximum number of minutes that a route can be suppressed
regardless of how unstable it is. You can set the maximum suppression time to a value from 1 – 20000 minutes.
The default is four times the half-life setting. Thus, if you use the default half-life of 15 minutes, the maximum
suppression time is 60 minutes.
The following example shows how to change the dampening parameters.
FESX424 Router(config-bgp-router)# dampening 20 200 2500 40
This command changes the half-life to 20 minutes, the reuse threshold to 200, the suppression threshold to 2500,
and the maximum number of minutes a route can be dampened to 40.
Summary of Contents for FastIron Edge Switch X424
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