Neighbour states
Two routers go through these states on the way to sharing their maps:
| State | What's happening |
|---|---|
| Down | No hello heard yet |
| Init | A hello arrived, but it doesn't list me yet |
| 2-Way | We both see each other in our hellos |
| ExStart / Exchange | We agree who goes first, then swap summaries of our databases |
| Loading | I ask for the LSAs I'm missing |
| Full | Our databases match: we are adjacent |
What must match
Two routers only become neighbours if these agree:
- They are in the same subnet, with the same mask.
- The area is the same.
- The hello and dead timers are the same (10 and 40 seconds on Ethernet).
- The router IDs are different.
- Neither interface is passive.
- Authentication (if used) matches.
- The MTU matches: if not, they get stuck in ExStart/Exchange.
Process numbers and router priorities don't have to match.
Below, R2's link to R3 has a different hello timer, so R2 and R3 stop being neighbours. The fix is to make the timers match:
In NetLab
▶ R3 R3>enable R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/BDR 00:00:37 10.0.13.1 GigabitEthernet0/0 R3#show ip ospf interface g0/1 | include Timer Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 ▶ R2 R2>enable R2#show ip ospf interface g0/0 | include Timer Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5 R2#show ip ospf interface g0/1 | include Timer Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5 R2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R2(config)#interface g0/1 R2(config-if)#no ip ospf hello-interval *Mar 1 00:00:09.000: %OSPF-5-ADJCHG: Process 1, Nbr 3.3.3.3 on GigabitEthernet0/1 from LOADING to FULL, Loading Done R2(config-if)#end *Mar 1 00:00:10.000: %SYS-5-CONFIG_I: Configured from console by console ▶ R3 R3#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 1.1.1.1 1 FULL/BDR 00:00:38 10.0.13.1 GigabitEthernet0/0 2.2.2.2 1 FULL/DR 00:00:37 10.0.23.1 GigabitEthernet0/1 *Mar 1 00:00:09.000: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on GigabitEthernet0/1 from LOADING to FULL, Loading Done
Tip: Changing the hello interval also changes the dead interval (to four times the hello) unless you set the dead interval yourself.
Remember: No neighbour? Check, in this order: is the link up, are both in OSPF (
show ip ospf interface brief), same subnet and mask, same area, same timers, not passive, different router IDs.