Thursday, February 12, 2009

OSPF on unnumbered links

I was reviewing the OSPF chapter in the CCIE exam guide today and something irked me. It said that OSPF neighbors will become adjacent if one or both of the neighbors are using unnumbered interfaces between them. I swear this was not case as I had experienced before so I labbed it up.
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
from FULL to DOWN, Neighbor Down: Interface down or detached

The adjcency 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

There we go! If one side is unnumbered, the other side needs to be also. I am running 12.4(7) so maybe this was not the case awhile ago, but right now it seems so. There are a few other mistakes in this chapter, especially in the beginning quiz - SO QUESTION (LAB) EVERYTHING!


  1. The unnumbered interface does not check the src ip address as an adjacency requirement.

    A PtP interface doesn't check the mask but it checks the ip src address.

    As long as the unnumbered interface is borrowing an address which falls within the farends subnet range an adjacency will be formed as both rules are being adhered too.
    So in your case you would need to modify lo0 to better match the farend.

  2. Mixing numbered and unnumbered interfaces makes for interesting scenarios ;) The details are described here (make sure you also check the links in the text).

  3. Excellent. I guess I was looking at it the wrong way. I was reading it as unnumbered/numbered could form an adjacency no matter what the IP addresses are. But they are still bound to the same subnet rule. Thanks!


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