OSPF Router ID

Lessons Discussion
31

Hello David

That sounds interesting. I tried labbing this up in Packet Tracer and found the same issue. I’m using Packet Tracer Version 8.0.0.0212.

I also found that once the router chooses the router ID, it cannot be changed even if you do a clear ip ospf process. Even when I manually configured the router ID, and then removed it, the manually configured router ID remained.

From my experimentation, I found that in order for OSPF to change the router ID, you must either reload the router, or delete the OSPF configuration and reinstate it.

At first I believed this was a bug, but for completeness, I tried the same thing on Cisco CML. It had the same behavior! :open_mouth:

What I have found is this:

  1. When the router ID has been chosen as the IP address of a physical interface, even if you create a loopback interface and clear the OSPF process, the router ID will not change. You must either reboot the machine or delete and recreate the OSPF config.
  2. When the router ID has been chosen as the IP address of a loopback, even if you create a new loopback with a higher IP address, clearing the OSPF process will not change the router ID. The only way it will change is if you delete the loopback interface or change its IP address to something else.
  3. When creating, changing, or deleting a manually configured router ID, the router ID will change after clearing the OSPF process.

Notice that it is only when you create, change or delete the router ID that you get the following message from the console:

% OSPF: Reload or use "clear ip ospf process" command, for this to take effect

So it seems that packet tracer is recreating the behavior of OSPF router IDs correctly. This has been interesting, thanks for pointing this out!

I hope this has been helpful!

Laz

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