OSPF Config questions

Unanswered Question
May 25th, 2007
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I have OSPF running locally at the main site.

The gateway router to the MPLS cloud has OSPF configured and there is a Loopback interface configured but the router id is set to the Ethernet interface.

My understanding is that you would want the loopback set to this.

Why would the config set the ethernet to the ID and not the loopback?

I know the loopback is configured as an endpoint for some IPSEC tunnels, would this be a reason why not to use it as the OSPF router ID?

We also have some 6509 switches that I configured loopback interfaces on, but the router ID is not set and is the ethernet interface at the moment.

Should I set it manually to be the ethernet?

If the power cycles, they will come up as the loopbacks as the ID.

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Overall Rating: 5 (1 ratings)
Richard Burts Fri, 05/25/2007 - 10:41
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Without knowing more about the configuration of the router and about the history and environment we can not tell why the Ethernet is the router id instead of the loopback. There are several reasons why the Ethernet may be the router ID instead of the loopback:

- perhaps the loopback did not exist or was shut down when the OSPF process started. In this case it would choose the Ethernet.

- perhaps the OSPF process is configured to use the Ethernet instead of the loopback.

- perhaps there is more than 1 OSPF process on the router. Each OSPF process must have a unique router ID. If one process chose the loopback as router ID then the other process must choose some other interface and may have chosen the Ethernet.

As long as the router ID is unique I do not believe that there is much reason for you to be concerned which interface it is.

I would not think that there would be any relationship between the loopback being used for IPSec tunnel peering and the OSPF router ID, but perhaps someone thought there was (or should be) a relationship and configured it that way.

If there have been configuration changes (changing IP addresses on interfaces, adding or removing loopback interfaces, adding or removing subinterfaces) when you power cycle the router it is possible that the OSPF router ID may change. It should not be a big deal whether it changes or not.




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