unnumbered interfaces and routing protocols/connectivity

kpjunglekpjungle Senior MemberMember Posts: 426
Hi All,

I am going through the ISCW lab portfolio (lab 3-12 troubleshooting lab) and I came across the concept of unnumbered interfaces. Basically you can borrow an IP from another interface. What kind of rocked my world a bit, was the fact that routing protocols would form over these links even if the subnet was different :/

R1 ( and R2 ( (both mask /24) can and will form an EIGRP adjacency in my case. Anyone done some reading on this? I thought the subnet had to match up for them to be equal.

I can somewhat reason that even though R1 doesnt have an interface with the net, I can create a static route pointing traffic towards that net to a tunnel interface, because it will then send traffic through that interface no matter what, but the whole scenario just seems to rock my world when things are not on the same subnet.. Basically they connect to each other on different subnets.. You can only do the unnumbered on point to point interfaces though.

Anyone able to shed some light on this?
Studying for CCNP (All done)


  • cisco_troopercisco_trooper Member Posts: 1,441 ■■■■□□□□□□
    You can also use a /31 on point-to-points....
  • kpjunglekpjungle Senior Member Member Posts: 426
    You can also use a /31 on point-to-points....

    That I can understand, but thats still on the same subnet however...
    Studying for CCNP (All done)
  • networker050184networker050184 Went to the dark side.... Mod Posts: 11,962 Mod
    When you use ip unnumbered the source address will be and not the actual ip.
    interface Loopback1
     ip address
    interface Serial1/2
     ip unnumbered Loopback1
    router ospf 1
     network area 0
    *Mar  1 00:06:29.819: OSPF: Send hello to area 0 on Serial1/2 from  <======
    An expert is a man who has made all the mistakes which can be made.
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