Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 460)

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time a route becomes unreachable or is withdrawn by a BGP4 UPDATE from a neighbor, the route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.
To enable route flap dampening using the default values, enter the following command.
device(config-bgp-router)# dampening
Syntax: [no] dampening [ half-life reuse suppress max-suppress-time ]
The half-life parameter specifies the number of minutes after which the penalty for a route becomes
half its value. The route penalty allows routes that have remained stable for a period 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. A dampened
route that is no longer unstable can eventually again become eligible for use. You can configure the
half-life to be from 1 through 45 minutes. The default is 15 minutes.
The reuse parameter specifies how low a penalty for a route must be before the route becomes
eligible for use again, after being suppressed. You can set the reuse threshold to a value from 1
through 20000. The default is 750 (0.75, or three-fourths, of the penalty assessed for a one flap).
The suppress parameter specifies how high the penalty for a route can be before the device
suppresses the route. You can set the suppression threshold to a value from 1 through 20000. The
default is 2000 (more than 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 through 255 minutes. The default is 40 minutes.
Globally configuring route flap dampening
460 FastIron Ethernet Switch Layer 3 Routing Configuration Guide
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