unnumbered interfaces and routing protocols/connectivity

kpjungle

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 (172.17.1.1) and R2 (172.17.2.1) (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 172.17.2.0/24 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?

cisco_trooper

You can also use a /31 on point-to-points....

kpjungle

cisco_trooper wrote:

You can also use a /31 on point-to-points....

That I can understand, but thats still on the same subnet however...

networker050184

When you use ip unnumbered the source address will be 0.0.0.0 and not the actual ip.

interface Loopback1
 ip address 1.1.1.1 255.255.255.0
!
interface Serial1/2
 ip unnumbered Loopback1
!
router ospf 1
 log-adjacency-changes
 network 1.1.1.1 0.0.0.0 area 0

*Mar  1 00:06:29.819: OSPF: Send hello to 224.0.0.5 area 0 on Serial1/2 from 0.0.0.0  <======