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Solution<br />

Route flap damping is a way to prevent flapping routes from destabilizing BGP. In<br />

the JUNOS software, you set up damping by using routing policy. There are four<br />

steps in setting up damping:<br />

1. Define the damping parameters:<br />

[edit policy-options]<br />

aviva@Router3# set damping damping-normal suppress 6000<br />

aviva@Router3# set damping standard-damping half-life 15<br />

aviva@Router3# set damping standard-damping reuse 3000<br />

aviva@Router3# set damping standard-damping max-suppress 30<br />

2. Create a routing policy that references the damping parameters:<br />

[edit policy-options]<br />

aviva@Router3# set policy-statement damping-policy from route-filter 10.0.31.1/32<br />

exact<br />

aviva@Router3# set policy-statement damping-policy then damping damping-normal<br />

aviva@Router3# set policy-statement damping-policy then accept<br />

3. Enable route flap damping for BGP:<br />

[edit protocols bgp]<br />

aviva@Router3# damping<br />

4. Apply the damping policy to a BGP group:<br />

<strong>Discussion</strong><br />

[edit protocols bgp]<br />

aviva@Router3# set group session-to-AS65505 import damping-policy<br />

If a link on the network is intermittently failing, routes can be withdrawn and readvertised<br />

in quick succession as the link goes down and then comes back up. This<br />

route flapping forces BGP to change any next hops that use the failed interface each<br />

time the link goes down. BGP then has to update its routing tables and propagate the<br />

new routing information. If many routes are being recalculated, the flapping link<br />

could make BGP very unstable.<br />

Route damping is a mechanism for preventing flapping routes from destabilizing a<br />

BGP network. Damping slows or stops the “vibrations,” or rapid changes, in the<br />

routing table. When a route flaps, it is given a specified number of demerits. The<br />

route’s accumulated demerits are reduced over time according to a configured decay<br />

rate. If the route’s accumulated demerits exceed a configured threshold, the route is<br />

suppressed until the number of demerits decays below a second configured threshold.<br />

Route damping is most useful in large service provider networks that have many<br />

attached peers and that carry many prefixes—a scenario in which the chances of one<br />

or more routes flapping at any given time is high.<br />

In the first part of the configuration, you set four damping parameters that are used<br />

to calculate a figure of merit, which controls how long a route can be suppressed. The<br />

458 | Chapter 13: BGP<br />

This is the Title of the Book, eMatter Edition<br />

Copyright © 2008 O’Reilly & Associates, Inc. All rights reserved.

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