Why use a loopback address/subnet as an OSPF area id?

rafaelti1 · ‎05-20-2015

I noticed at my job on some of the device configs, they use a loopback address/subnet as the actual area id for OSPF.

So instead of your usual OSPF network command like

router ospf 201

network 192.168.0.0 0.0.255.255 area 0

there is,

router ospf 201

network 192.168.0.0 0.0.255.255 area 10.2.2.1

The CCNA and CCNP books I have read they don't ever mention configuring OSPF this way and in the production network at my job I have seen OSPF configured both ways, with the traditional area id and the loopback, so you can imagine how this can be a little confusing. When coming from a traditional Cisco point of view.

Why would you configure it this way and how can I understand it better?

Thanks,

Joseph W. Doherty · ‎05-20-2015

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Posting

That's likely not a loopback IP, it's more likely just a dotted decimal representation of an area number. I.e. just a "nicer" way to list a large area number (10.2.2.1 = 167903745). Might be used to correspond to an IP address block assigned for area usage.

rafaelti1 · ‎05-31-2015

I agree with the possibility of it corresponding to an IP block. I have no idea why it was set up like that as we dont have insane area numbers. I will read up on some of the topology maps to try and make sense of it. I guess in the end as long as it connects to "area 0" it should not matter then. Would you agree?

Joseph W. Doherty · ‎05-31-2015

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yep, actual area number, besides zero, has no special meaning to OSPF.

Mark Malone · ‎05-20-2015

Thats just setting ospf area decimal rather than ip format so area 0 would be just 0.0.0.0 in decimal , Cisco provide the option to do both not sure why

Heres an example from another page and a link may help

For example, convert area 400 decimal into binary (i.e. 110010000). The binary number needs to show 32 digits, representing the digits of the 4 octets in the IP type format. So, 110010000 binary = 00000000.00000000.0000001.10010000 as broken into octets. Finally, convert each octet into a decimal value. (e.g. 00000000.00000000.0000001.10010000 = 0.0.1.144). Therefore, 400 = 0.0.1.144

https://cciepursuit.wordpress.com/2007/09/15/dotted-decimal-ospf-areas/

Peter Paluch · ‎05-20-2015

Hi Mark,

Cisco provide the option to do both not sure why

I suppose it is simply because the area number is a 4B number, and especially if you wanted to use area numbers higher than 255 for whatever reason, using dotted decimal notation is more readable to humans. Some other vendors, for example, accept OSPF area IDs only in dotted decimal format. To the router, it's the same 32 bit quantity, just it is presented in a different format.

Best regards,
Peter