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JUNOS Cookbook by Aviva Garrett

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12.12. Improving OSPF Convergence Times

Problem

You want to speed up convergence of OSPF routes in case a path fails with no hardware indication, which might happen on an Ethernet network.

Solution

Reduce the interval at which OSPF exchanges Hello messages and, in parallel, decrease the dead interval:

	[edit protocols ospf area 0.0.0.0]
	aviva@RouterG# set interface fe-0/0/1 hello-interval 2
	aviva@RouterG# set interface fe-0/0/1 dead-interval 8
	aviva@RouterG# set interface fe-1/0/1 hello-interval 2
	aviva@RouterG# set interface fe-1/0/1 dead-interval 8

Discussion

The OSPF protocol specifications were developed when routers and network interfaces were slower to allow enough time for LSAs to reach all nodes and for the SPF calculation to run on all routers. Modern routers and interfaces are much faster, so one strategy for speeding up route convergence is to modify the default OSPF timers. The base JUNOS OSPF code already optimizes convergence times by doing fast link detection and flooding, by quickly regenerating LSAs, and by quickly scheduling SPF calculations, so you don't often need to modify the timers.

When changing OSPF timers, you must modify each interface, changing the hello timer and dead timer intervals. The default timer settings, as defined in the OSPF specification, are 10 seconds for sending periodic Hello packets and 40 seconds for declaring the adjacency dead, or down (four times the hello interval):

	aviva@RouterG> show ospf interface detail Interface State Area DR ID ...

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