What are the OSPF hello and dead timers, and what are the defaults?
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Hellos discover neighbors and keep them alive. The dead interval is how long a router waits without a hello before it declares the neighbor down. Defaults are 10 and 40 seconds on broadcast and point-to-point networks, and 30 and 120 seconds on NBMA and point-to-multipoint. Both must match between neighbors.
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Defaults by network type (IOS)
| Network type | Hello | Dead |
|---|---|---|
| Broadcast | 10 s | 40 s |
| Point-to-point | 10 s | 40 s |
| Non-broadcast (NBMA) | 30 s | 120 s |
| Point-to-multipoint | 30 s | 120 s |
The dead interval defaults to four times the hello. If you change only the hello, IOS also moves the dead interval to four times the new hello unless you set dead explicitly.
Changing them
interface GigabitEthernet0/0/0
ip ospf hello-interval 5
ip ospf dead-interval 20Sub-second detection with OSPF alone uses ip ospf dead-interval minimal hello-multiplier 4, which sets
dead to 1 second and sends 4 hellos per second. In practice I use BFD instead, which is lighter on the CPU.
What else hellos carry
Router ID, area ID, mask, hello and dead intervals, options (including the stub bit), router priority, DR and BDR addresses, and the list of neighbors already heard on the link.
They will ask next
What happens if hello timers do not match?
The router drops the hello and the neighbor never appears.
debug ip ospf helloshows a "mismatched hello parameters" message naming the received and configured values.Why not just set the hello to 1 second everywhere?
It costs CPU on every router and every link, and hellos are processed by the control plane, so a busy CPU can miss them and cause false flaps. BFD runs in a lighter path and often in hardware.
Traps
- Thinking only the hello must match. The dead interval must match too.