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 18.104.22.168 0 FULL/ - 00:00:37 192.168.23.2 Serial1/1 22.214.171.124 0 FULL/ - 00:00:39 192.168.34.4 Serial1/0 R3# R3(config)#int s1/0 R3(config-if)#ip unnumbered lo 0 R3(config-if)# *Mar 2 06:31:01.600: %OSPF-5-ADJCHG: Process 1, Nbr 126.96.36.199 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 R4(config-if)# *Mar 2 06:33:14.288: %OSPF-5-ADJCHG: Process 1, Nbr 188.8.131.52 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.