OSPF Failure Detection

The OSPF hello packet has a secondary function: it can detect when a neighbour is unreachable or in a poor state.

OSPF has a second timer called the dead interval timer, which by default is 4 times the length of the hello timer. When a hello packet is received, the dead timer resets.

If a hello packet is not received for the entire duration of the dead timer, the state of the neighbour is set to Down, and a LSA is sent out advising of the topology change. This causes the SPF algorithm to recalculate metrics and routes in the area.



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