I need help answering the following question... I am running OSPF on my router's, and I want to use Cisco's router-id command to set the router-id of my OSPF process manually. Most documentation says to set this to a loopback address (or it will automatically use the highest loopback).
My question is if I use the actual IP of the interface as my router-id, and that interface goes down, does OSPF restart and does a DR election reoccur (this is a broadcast network)? Or does the router-id command allow for OSPF to remain stable even if the specified IP configured on an interface goes down?
If an OSPF process is using a physical interface address as its router ID, and that interface goes down, that address is no longer reachable. OSPF keeps running, but may lose connectivity to its neighbors until the interface comes back up.
Using a loopback address solves this problem, as it should always be up and reachable from all physical interfaces.
"To use a fixed router ID, use the router-id command in router configuration mode. To force OSPF to use the previous OSPF router ID behavior, use the no form of this command.
You can configure an arbitrary value in the IP address format for each router. However, each router ID must be unique.
If this command is used on an OSPF router process which is already active (has neighbors), the new router-ID is used at the next reload or at a manual OSPF process restart. To manually restart the OSPF process, use the clear ip ospf command."
However, as jstretch pointed out, we usually define a loop back interface and assign it an IP. The router will be forced to always use this IP as it's ID for OSPF no matter what physical interfaces are up or down.