Solved: Mixed/Overlapping VLSM for OSPF

grandcanuck · ‎02-12-2016

I have a simple question I'm having difficulty finding an answer to. Can I use overlapping VLSM statements to break off only one (or a few) /30s in a different area? Something like:

network 192.168.253.0 0.0.0.255 area 0

network 192.168.253.64 0.0.0.4 area 91

(notice how these overlap). I have the first line there which puts all the /30s in a single area. I need to break them off in a different area.

Regards,

Rolf Fischer · ‎02-13-2016

Hello and welcome at the CSC forum!

Yes, you can use overlapping network statements for different areas under the same OSPF process. You can find a more detailed answer in this discussion: https://supportforums.cisco.com/discussion/12510021/conflict-router-ospf-netwok-commands

network 192.168.253.64 0.0.0.4 area 91

I guess there is a typo in the wildcard mask? The IOS OSPF implementation does not allow discontiguous wildcard masks in network statements. You can only use 0.0.0.0, 0.0.0.1, 0.0.0.3, 0.0.0.7 and so on.

HTH
Rolf

View solution in original post

Rolf Fischer · ‎02-13-2016

Hello and welcome at the CSC forum!

Yes, you can use overlapping network statements for different areas under the same OSPF process. You can find a more detailed answer in this discussion: https://supportforums.cisco.com/discussion/12510021/conflict-router-ospf-netwok-commands

network 192.168.253.64 0.0.0.4 area 91

I guess there is a typo in the wildcard mask? The IOS OSPF implementation does not allow discontiguous wildcard masks in network statements. You can only use 0.0.0.0, 0.0.0.1, 0.0.0.3, 0.0.0.7 and so on.

HTH
Rolf

grandcanuck · ‎02-17-2016

Thanks, I indeed fat fingered the mask, meant 0.0.0.3. I did run tests and it works fine. I was able to add a network statement for the /24 and a few different ones for specific /30's.

Thank you.

Joseph W. Doherty · ‎02-17-2016

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As you've found, and as Rolf explained, it can be done.

Years ago, you had to be careful with how you sequenced your network statements, for example, in your OP you have the larger address block first, so none would make it to the more specific. In later devices (or IOS versions), I believe OSPF will reorder the overlapping network statements, so the more specific is matched first.