Let's get our interfaces configured and a simple OSPF AREA 0.
Now let's take a look at some default OSPF settings specifically the hello / dead timer which is one of the criteria that must match in-order for OSPF neighbors to form adjacency.
Notice that the hello timer is set to 10 seconds and dead interval is set to 4 x hello interval (10) so 40 seconds. This means that every 10 seconds a hello message will be sent to the multicast address 188.8.131.52 (multicast address for all OSPF routers). If 4 hellos are missed from a neighbor then the neighbor adjacency will be considered down. This means by default it may take up to 40 seconds before realizing that a neighbor is down.
|The timer will keep refreshing as long as it is getting hellos from the neighbor.|
RTT average is 28ms so roughly 14ms (one way latency) for a packet to reach R1. Using that as our baseline let's configure OSPF fast hellos.
Notice that all routers in a segment must have the same dead / hello timer, therefore make sure to apply this command all routers involved. This command basically sets the hello timer to dead timer / hello multiplier. So dead timer (1 second) / hello multiplier (4) = 250 msec. So every 250 msec a hello will be sent totaling 4 hellos per second. So if 4 hellos are missed in a row then neighbor will be considered down rather than wait 40 seconds or 4 seconds.
Fast hellos is a great tool to fine tune OSPF, however with most tuning parameters please make careful considerations before implementing it.
Many more articles to come so stay tuned.
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