OSPF determines router id in one of three ways. The following list shows the determination mechanism:
- If an ID has been specified with the router-id command, use it.
- If there is no router-id command, use the highest numbered loopback interface, regardless of the physical interface numbers.
- If there is no router-id set, and there are no loopback interfaces, use the highest numbered physical interface as the OSPF router id.
So, what happens if none of these conditions are met? (A better question might be, “Why would you run OSPF on a router with no IP addresses set on it?” I’ll work an answer to that one into my in-progress treatise, 501 Dumb Thing To Do with your Network Lab.)