clarification on RID for OSPF for test?

Unanswered Question
Sep 21st, 2007

I am reading conflicting information on how OSPF RID is selected. They all agree it starts by choosing a specified RID, then failing that, the highest loopback IP. Then failing that...they all say highest IP but do not agree on if that includes interfaces that do or do not participate in OSPF. I'm reading BA materials plus CP's authorized self study as well as official guide.

SO...if you HAVE to go as far down as looking at interfaced to get the highest IP to use as RID...

Which is it selected from: ALL interfaces that are up or only those interfaces that are both up and participating in OSPF process?

Is the real life answer different from what is expected on test?

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john.jay.franklin Sat, 09/22/2007 - 13:34

Here is how it is selected:

1) Specified RID in OSPF configuration (router-id 1.2.3.4)

2) Highest loopback address (if any)

3) Highest active IP address (active means not shutdown. Does not have to be participating in OSPF to be considered)

Hope that helps. :)

Kevin Dorrell Sat, 09/22/2007 - 21:21

Just to add to that, older IOS versions have a nasty behavior regarding option (3) - it used to be that if the interface supplying the RID got shut down or re-configured, then the RID would change on the fly, making the OSPF re-converge. (Yes, even if that interface was not participating in the OSPF.) Thankfully it does not do that any more, and once the RID has been decided, it sticks to it. Not sure which version changed that.

Kevin Dorrell

Luxembourg

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