OSPF behavior on unnumbered links

By | December 29, 2021

I saw few books says that OSPF neighbors will become adjacent if one or both of the neighbors are using unnumbered interfaces between them. But I observed completely opposite behavior when I checked it in lab.

R3#sho ip ospf ne

Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/  -        00:00:37    Serial1/1           0   FULL/  -        00:00:39    Serial1/0

R3(config)#int s1/0
R3(config-if)#ip unnumbered lo 0
*Mar  2 06:31:01.600: %OSPF-5-ADJCHG: Process 1, Nbr on Serial1/0

The adjacency will not come back up. Let’s configure R4:

R4(config)#int s1/1
R4(config-if)#ip unnumbered lo 0
*Mar 2 06:33:14.288: %OSPF-5-ADJCHG: Process 1, Nbr on Serial1/1
from LOADING to FULL, Loading Done

So It means both side should be unnumbered to be for the adjacency. However i was using IOS version12.4(7). So maybe this was not the case awhile ago, but right now it seems so.

Thanks for reading the blog.

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