The OSPF hello timer sets the interval between hello messages sent by an OSPF-enabled interface. Hello messages discover neighbors and maintain established neighbor relationships. On broadcast and point-to-point networks, the default interval is 10 seconds; other network types use 30 seconds. section 18.3.1 section 18.3.2
The hello timer works with the dead timer. With the usual 10-second hello and 40-second dead-timer defaults, a router removes a neighbor after receiving no hello for 40 seconds. A physical interface failure removes the neighbor immediately instead of waiting for the timer. section 18.3.1 section 18.3.2
Check these OSPF hello-timer facts:
- Match the hello and dead timers on both interfaces; a mismatch can bring an established adjacency down. section 18.3.3
- Use the interface command `ip ospf hello-interval seconds` to change the interval. section 18.3.3
- Remember that the hello timer is distinct from the STP hello timer, which controls BPDU transmission and defaults to 2 seconds. section 14.4.2
A timer mismatch takes the adjacency down
R2(config)# interface g0/0 R2(config-if)# ip ospf hello-interval 5 %OSPF-5-ADJCHG: ... from FULL to DOWN, Neighbor Down: Dead timer expired R2(config-if)# ip ospf hello-interval 10 %OSPF-5-ADJCHG: ... from LOADING to FULL, Loading Done Changing R2 from the 10-second default to 5 seconds causes its adjacency with R1 to fail. Restoring 10 seconds allows the routers to become fully adjacent again.
