I configured a couple of routers in my lab on an ethernet broadcast segment. I didn't blow away one router config and it still had an interface configured for 192.168.1.1. I added a network between the two routers (R1 and R5) of 220.127.116.11 /24 and placed two interfaces into area 0. I also added some loops (R1 is 18.104.22.168 and R5 is 22.214.171.124) to area 0. So when the neighbor relationship formed, R1 indicated that its neighbor's RID was 192.168.1.1. This didn't make any sense to me at all since a loopback interface, part of the OSPF process or not, always takes precedence for RID over an active physical interface (again, part of OSPF or not). So I shut the interface down and cleared the process on both routers. The neighbor RID of 192.168.1.1 persisted. It wasn't until I went into R5 and REMOVED the ip address on the interface and reset the process on both routers that it finally showed the correct neighbor ID of 126.96.36.199 (according to the RID rules). Attached is output of the occurrance on my routers. Anyone care to take a guess as to what was going on? It's important to me because I thought for sure I had a solid understanding of the RID rules as follows:
1. Highest priority wins - priority is '1' by default on all routers, so we need a tie breaker...highest RID
2. RID breaks down thusly:
A. Configuration of 'router-id' on the OSPF process has highest weight
B. Highest Loopback interface configured, active, and part of the OSPF process or not
C. If no active Loopbacks, highest active physical interface configured (again, part of the OSPF process or not)
So I could see how what I experienced would be correct if it were the fact that it didn't matter if the interface was up or not. But my understanding was always that if the interface was administratively shutdown that it wouldn't be considered.