Solved: Advanced OSPF routing with multiple paths and forcing certain traffic . .

jecker · ‎05-16-2008

I basically have a 3 site triangle configuration all in OSPF area 0. Two of the sites are connected via T1 and the other link is connected via fiber. I added an OSPF cost to one of the T1's (call is backup T1) so that traffic never get routed accross it. However, I would like to force a small subset of traffic accross is this backup T1.

I know I can use route maps to accomplish this however i'd like to use OSPF in the event that the backup T1 goes down, this small subset of traffic will still be routed the alternitive route.

Is there a way in OSPF to force only a small subset of traffic to route accross a lower cost link, but will still route accross the primary T1 if the backup is down.

Thanks.

Richard Burts · ‎05-16-2008

Jesse

There are parts of your question that I do not understand very well. Let me answer what I think I do understand and if that is not the full answer that you need then perhaps you can clarify the rest for me.

There is not any way in OSPF to route a small subset of traffic differently. OSPF determines routes and all traffic is treated the same on these routes. If you need to route a small subset of traffic differently then you need Policy Based Routing, which uses route maps. In PBR you can identify certain types of traffic and can choose to forward it differently from what the routing table would do. With PBR if the next hop that you are using becomes not available then PBR will let the traffic be routed in the normal way. It sounds to me like this is a solution that comes pretty close to what I understand of your requirements.

HTH

Rick

HTH

Rick

View solution in original post

Richard Burts · ‎05-16-2008

Jesse

There are parts of your question that I do not understand very well. Let me answer what I think I do understand and if that is not the full answer that you need then perhaps you can clarify the rest for me.

There is not any way in OSPF to route a small subset of traffic differently. OSPF determines routes and all traffic is treated the same on these routes. If you need to route a small subset of traffic differently then you need Policy Based Routing, which uses route maps. In PBR you can identify certain types of traffic and can choose to forward it differently from what the routing table would do. With PBR if the next hop that you are using becomes not available then PBR will let the traffic be routed in the normal way. It sounds to me like this is a solution that comes pretty close to what I understand of your requirements.

HTH

Rick

HTH

Rick

jecker · ‎05-16-2008

Thanks for the rely. You answered the questions I was looking for.

I did know that route maps would accomplish this task, I knew that I could track the next hop's reachability, but I wanted to find out if OSPF could do this or now.

I did not realize that a route map was like a static route, in the fact that it would not be used if thet next hop was down.

Thanks again

Giuseppe Larosa · ‎05-16-2008

Hello Jesse,

I think that Rick has given you the correct answer: OSPF is a routing protocol and there is no way to route a subset of traffic out of the shortest path (OSPF = Open Shortest Path First).

I f you want to use the backup T1 only for some traffic you need PBR that can perform as you like (including using the normal routing table if the next-hop is not available).

In the original design of OSPF there was a possibility to have TOS based routing (different link costs for different TOS values) but this is not currently implemented.

hope to help

Giuseppe

Richard Burts · ‎05-16-2008

Jesse

I am glad that my response was able to resolve your question. Thank you for using the rating system to indicate that your question was resolved (and thanks for the rating). It makes the forum more useful when people can read a question and can know that there was a response which did resolve the question.

The forum is an excellent place to learn about Cisco networking. I encourage you to continue your participation in the forum.

HTH

Rick

HTH

Rick