ospf adjacency

Unanswered Question
Feb 8th, 2010

I have an issue and I think I know the answer but I thought I would run it by you all.

I'm implementing an ospf network and I'm not able to establish an adjacency with one router. Now the router has an interface with a /16 network, whereas everything else has various subnets of that /16. Is this the cause?

I know I could just re-subnet that mask, but that in itself causes problems (not related to ospf).

Is there a way around this? I must admit I didn't think about this much before I started as it's just been dumped on me.

I have this problem too.
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ryan.lambert Mon, 02/08/2010 - 07:35

Where you could be running into an issue is, the IP address on the router with the /16 mask does not fall within the subnet boundaries of the other routers chatting OSPF (ie: in the network x y area z statement).

Could you give us some background on why it's like this and what the perceived impact is?

I would suggest something like creating a different, dedicated transit network to establish your adjacencies on... but it's tough to make that recommendation in confidence without understanding the background here. If you do get that adjacency working and the other routers start sharing more specific prefixes, it may cause some undesirable behavior accessing machines in the network with the larger mask.

Example: if you had something on the 'large network' numbered within the boundaries of the 'smaller networks', short of a static route pointing to the correct router, machines in the smaller networks would be unlikely to take the path to 'large network' because there is a more specific route to follow. Similarly, even if they are in the same VLAN and the subnet masks are mismatched, machine A may think machine B is local, but the feeling isn't guaranteed to be mutual.

Hope that makes sense - it's Monday.

codflanglers Mon, 02/08/2010 - 07:40

Thanks for the reply. I thought that was the case. The reason it's like this is this used to be one flat network (up to last year, when I arrived and subnetted everything). The servers all point to that IP and mask as their default gateway. I am changing them as we speak but some are going to be tricky. I'm moving to OSPF over static routes (of which there are hundreds).

I was either going to create a new network and get the adjacency that way or create more ospf areas.

ryan.lambert Mon, 02/08/2010 - 07:48

Gotcha.

Fixing the mask is probably the best long-term solution, anyway.

IMO, better than starting a project to clean up dirt and creating even more confusing dirt in the process.

codflanglers Mon, 02/08/2010 - 08:00

Yeah, I figured that was going to be the case. Trouble is not changing the mask, but moving all the services off it. Can open....worms everywhere

Giuseppe Larosa Mon, 02/08/2010 - 09:32

Hello Codflangers,

Proxy ARP can help in this kind of migration: it allows to help end user devices or servers that have a less specific  mask then the one effective: when they send ARP requests for hosts they think are in subnet the router answers with its own MAC address.

you can enable it with ip proxy-arp in interface configuration mode.

In this way you can put the right mask on the router so that it can build the OSPF adjacency and it can serve devices using the old short mask.

Hope to help

Giuseppe

ryan.lambert Mon, 02/08/2010 - 10:34

That is a good point.

The only thing I would caution is along the lines of what I mentioned earlier...

If hosts that have been using the shorter (/16) mask fall within the boundaries of your longer ones (/24 or whatever) for some reason, once that mask is fixed and the adjacency is formed, you may find some head-scratcher issues like traffic not making it to certain hosts, even with proxy-arp enabled. As part of your clean-up I am assuming that you are going to purge all the static routes, so if there is anything that is configured as a static route (host route, maybe) to patch this current environment together, you may see that kind of strange behavior once they are gone.

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