OSPF router ID query-Un used rtr IDs

[fortis123]

Hi all,

We have 6513 as core rtrs in our infrastructure. The 'Show ip ospf' o/p showing the rtr ID as :192.168.101.1 on S1 and 192.168.101.2 on S2.

But in our environment we are not using 192.168.101.0 subnet at all.

Also, in the OSPF config , no rtr ID set.

Lo0 interfaces 'UP' with no ip addresses.

no 192.168.101.0 advertised in OSPF.

6513-S2#sh ip ospf

Routing Process "ospf 100" with ID 192.168.101.2

6513-S1#sh ip ospf

Routing Process "ospf 100" with ID 192.168.101.1

This infrastructure has been setup long

time back, do not want to modify anything but curious to know hopw this IP is showing as router ID as whaen it is not existing at all in the config.

Please suggest.

Thank you

MS

[Richard Burts]

MS

There might be several possible explanations for what you describe:

- it is possible to configure a router ID for the OSPF process and if a router ID is manually configured it is possible to use any IP address you wish and it does not need to be an address on the router. Is it possible that a router ID was configured at some point (and then perhaps removed some time later)?

- if there is not a configured router ID then the OSPF process will choose an address to use as router ID. It will prefer a loopback address and if no loopback address is available then it will choose an address from another interface. Is it possible that 192.168.101 existed on these devices at some point (even if it was subsequently changed or removed)?

Perhaps you could post the output of show ip interface brief?

HTH

Rick

[fortis123]

Hi Thank you. Iam thinking 2nd might have happened. Will this cause any issues..?

Thank you

MS

[Richard Burts]

MS

While it is not an optimal situation, I do not believe that it will cause any issues.

HTH

Rick

[Thotsaphon Lueangwattanaphong]

Hi MS,

As rick stated,It's not a normally situation. OSPF should use the ip address of router-id command or a highest active interface to be a RID.

For testing : Can you use "clear ip ospf process" command? then see what's going on.

Hopes this helps

Thot

[mohammedmahmoud]

Hi,

If the RID was selected based on an interface that existed before, as a general rule the router continues to use it even if the interface subsequently fails or is deleted, to change this you either need to configure the RID manually or reload the router (clearing the ospf process won't release it). I don't recommend leaving the case as it is now, since this can affect any configuration done using the RID (virtual-links for example) on the next router reload (since the router will take a new RID, since the interface that was used before is not there anymore). Finally i recommend always configuring the OSPF RID manually to prevent any hassles.

BR,

Mohammed Mahmoud.

[Thotsaphon Lueangwattanaphong]

Hi Mohammed,

I lab it up and you're right! RID still not be changed. Configuring router-id command on router process and after that we still need restarting ospf process or reloading router as well.

Thanks

Thot

[mohammedmahmoud]

Hi Thot,

You are very welcomed.

BR,

Mohammed Mahmoud.