The OSPF dead timer is the interval a router waits after receiving no hello message before removing that neighbor from its neighbor table. The default is 40 seconds, while the default hello timer is 10 seconds. section 18.3.1
The timer maintains an established OSPF neighbor relationship even after routers reach Full state. It matters when an interface remains up but hellos stop arriving; a physical link failure removes the neighbor immediately instead. section 18.3.1 section 18.3.2
Use these operational and exam-focused points:
- Match hello and dead timers on both interfaces; a mismatch prevents the routers from forming or maintaining the adjacency. section 18.3.3
- Expect 10-second hello and 40-second dead defaults on broadcast and point-to-point OSPF network types; other network types use 30- and 120-second defaults. section 18.3.2
- Interpret Dead Time in show ip ospf neighbor output as the remaining time before the neighbor is removed if no hello arrives. section 18.3.1
Configuring and troubleshooting the timer
On an OSPF-enabled interface, use ip ospf dead-interval seconds to change the dead timer. For example, interface configuration can set ip ospf dead-interval 40. Verify the neighbor relationship with show ip ospf neighbor. If a timer mismatch occurs, the adjacency can move from Full to Down with a Dead timer expired message; restoring the matching value allows the routers to become fully adjacent again. section 18.3.3
R2# show ip ospf neighbor #1 Neighbor ID Pri State Dead Time Address Interface 172.16.1.1 1 INIT/DROTHER 00:00:38 10.0.0.1 GigabitEthernet0/0#2 #1 Use this command to display the OSPF neighbor table. #2 R2 has an entry for R1 in the Init state.
Inspect a representative Cisco neighbor table showing OSPF state, remaining Dead Time, neighbor address, and interface. section 18.3.1
