Found out an interesting thing while doing a Vol3 lab. We all know that EIGRP will not form an adjacency over a non common subnet unless special circumstances. What about OSPF? I had a lab task where I need to form an adjacency over an ip unnumbered link. To my surprise this works just fine. If you debug you will get a message like this:
*Mar 1 00:15:14.155: OSPF: Rcv pkt from 155.1.15.2, FastEthernet0/0, area 0.0.0.0 : src not on the same network
However the database is correct and routes are installed. If you want to try this yourself use two routers with a serial link between them and then do ip unnumbered from an Ethernet interface and enable OSPF as usual. Watch what happens. Does anyone have a good description of this behavior? I tried looking in the RFC but all I could find is this:
“All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor relationships.”
“An adjacency is bound to the network that the two routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.”
