I was reading through some other CSC posts and I found one with a config which includes the IP unnumbered command. I didn't know what it did so I was interested, I started reading up on it a little and the below Cisco link:
I understand that this command effectively borrows an ip address from another interface, however what I don't understand is how it works with EIGRP adjacencies. On the above link about a 3rd of the way down the page is an example of IP unnumbered (entitled: Same Major Net different subnet) and it establishes an IGRP adjacency between two routers however the ip addresses are not in the same subnet:
Here is my question. Why does this establish an adjacency? I labbed it up and if I follow the configuration as per the example (except using EIGRP instead of IGRP) it works perfectly. However if I take ip unnumbered out of the equasion and just assign the ip addresses straight to the interface I get the error:
*Mar 1 00:25:22.715: IP-EIGRP(Default-IP-Routing-Table:1): Neighbor 18.104.22.168 not on common subnet for Serial0/0
Can anyone explain why it works when using ip unnumbered and not when applying the ip address direct to the interface?
Also if you have an example of why this would be useful in the real world that would be great, I'm not coming up trumps on this!