OSPF Failure Detection

Question 1

Failure Detection

Answer 1

A secondary function of OSPF hello packets is to ensure that adjacent OSPF neighbors are still healthy and available. OSPF sends hello packets at set intervals, according to the hello timer. OSPF uses a second timer called the OSPF dead interval timer, which defaults to four times the hello timer. Upon receipt of the hello packet from a neighboring router, the OSPF dead timer resets to the initial value, and then it starts to decrement again.

If a router does not receive a hello before the OSPF dead interval timer reaches 0, the neighbor state is changed to down. The OSPF router immediately sends out the appropriate LSA, reflecting the topology change, and the SPF algorithm processes on all routers within the area.