Advertise Class B network as Class C.

[polm]

I have a RIP v1 network of mulitiple Class B networks.

This RIP process redistributes these Class B networks to an OSPF process.

Can I manipulate the OSPF process to advertise these Class B's as Class C's?

For example, 172.16.0.0/16 really only has active nodes on 172.16.30.0/24. Even though the nodes are configured with a 255.255.0.0 subnet mask.

Would I break routability if I were to advertise this network as 172.16.30.0/24 via the OSPF process?

 

[spidey07]

I'm trying to think of a way to do this...

If you do a static to null0 and redistribute that into OSPF, the route will have the correct mask in OSPF but it won't get routed becauss then that null0 route would be the best route (shorter prefix length)

Can't you just "redistribution rip subnets" under the OSPF process? that should take care of it.

 

[polm]

It's these old RIP networks that are the problem, as they were all built on Class-B addresses. Even though they only use 2 - 25 addresses on each subnet. For example, I have a network with 5 nodes (172.16.33.5-10) that are all configured for 172.16.0.0/16.

These old RIP networks boundary our big OSPF network and are all redistributed into the routing tables. The plan is to manipulate the OSPF process to stop redistributing dynamically. Then we could define the routes into the RIP network ourself.

So I was thinking we could tell OSPF to advertise 172.16.33.0/24 instead of 172.16.0.0/16. This I can do, but I'm worried it will break routability to the RIP networks.

Once a node gets to the OSPF/RIP boundary and has been heading toward 172.16.33.5 via a Class-C route, it will come to the Class-B route. Will this cause a problem, or will the traffic pass OK?

 

[spidey07]

should be fine as long as you're not doing mutual redistribution. A better approach might just be statics if it is only a few networks.

how are you modifying the route?

 

[polm]

My plan is to kill the dynamic redistriubtion of RIP into OSPF. Then I will create static routes using the Class-C subnets to point to the Class-B RIP networks.

 

[cmetz]

polm, look at Cisco's conditional advertisement feature. You can make advertisement of route X depend on matching against route map Y. Assuming that they added this feature for OSPF (I've only used it for BGP), it should solve your problem.

 

[polm]

cmetz, unforutunately I'm working with ancient Nortel routers (ARN's and BCN's) at this RIP/OSPF boundary.

 

[polm]

I've tested the scneario and it works. Thanks all !

 

[spidey07]

cool, if it is only a few routes then I'd go the static route as well and just redistribute that.

no need to do redistribution if it isn't really needed.