Is it required that OSPF router-id must reachable in IGP cloud?

[bobbie0001]

Is it required that OSPF router-id must reachable in IGP cloud?

[Jon Marshall]

No it isn't.

Jon

[bobbie0001]

Hello Jan

Can you provide me a detailed explanation? Why it doesn't required?

[Jon Marshall · Accepted solution]

Not sure what to say to really.

The RFC specifies that each router in a router OSPF autonomous system must have a unique id.

On Cisco routers you can explicity set it using the "router-id <ip address>" under the OSPF configuration but if you don't then it uses the loopback with the highest IP and if there are no loopbacks then the physical interface with the highest IP.

But there is no requirement for this IP to be routable ie. it simply ensures that each router has a unique id as per the RFC.

If you wanted to be able to telnet/ssh to the router id IP then yes you would need to advertise that so you can route to the IP from a remote subnet.

Jon

[bobbie0001]

Thank you Jon

[Joseph W. Doherty]

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Posting

The router ID, is just that, a router (node) ID.  It needs to be unique within the topology, but it doesn't have to have anything to do with the actual topology IP addressing.  Although router IDs default to interface IPs, and we often use loopback interface IPs (including making them routable), you can also set the router ID within the OSPF process to be whatever you want it to be.