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Community Member

OSPF load balancing

Please see attachment for diagram and questions. Thanks, Khoa

8 REPLIES
Hall of Fame Super Gold

Re: OSPF load balancing

With CEF and OSPF the default is to do per destination load balancing.

There is no guarantee that return traffic will follow the same path that outbound traffic used.

HTH

Rick

Cisco Employee

Re: OSPF load balancing

2. If host H1 on network N initiates traffic towards Destinations and traffic happens to take the path

through RouterB, will the return traffic from Destinations to host H1 take the same path through RouterB?

As Richard stated on Q2, there is no guarantee that the returning traffic will take the same path. The CEF hash algorithm will be ran independently on RouterA (for traffic from H1 to destination) and on RouterB (for returning traffic) and a different link might be selected.

Hope this helps,

Harold Ritter
Sr. Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
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Community Member

Re: OSPF load balancing

Thanks for your responses. It certainly clears things up. But I am not ready to give up yet; is there anything I can do on those routers to make the returning path the same as the originating path?

Thanks again,

Khoa

Hall of Fame Super Gold

Re: OSPF load balancing

Khoa

I am not sure that it is worth it, but if you really want to try something to assure that traffic takes symmetric paths there is one possibility that occurs to me. You might think about implementing Policy Based Routing. PBR presents an opportunity to steer traffic in particular ways and to over-ride the normal routing decision. You could set up PBR to send certain types of traffic (perhaps certain source and destination combinations, or perhaps all HTTP, or FTP, or ...) over path 1. And set up PBR on the other end to send the corresponding traffic over path 1. And you could set up PBR to send certain other traffic over path 2. And set up PBR on the other end to send corresponding traffic over path 2.

Implementing PBR will require much careful analysis on your part. And the responsibility for how traffic is balanced between the links is now dependent on PBR and how you implement it. And to me it is not worth it. But if you want something that shares traffic over both links and that will send return traffic on the same path, then this is the alternative that I can think of.

HTH

Rick

Re: OSPF load balancing

Hi,

MPLS Traffic engineering would do the trick as well ... but that is overkill (though fun ;-).

Regards

Martin

Community Member

Re: OSPF load balancing

Thanks again for your replies.

The way this network is set up is that there was a load balancing scheme between RouterA and the pair (RouterB, RouterC). In other words, when host H1 connected, Router A may decide to use the path thru RouterB this time, but may select RouterC path the next time this same host trying to connect.

So, I guess the OSPF/MPLS/PBR/"others?" load balance on RouterD need to be aware of the interface where traffic start flowing in so that it can set up the return path to go out of the same interface. I am not sure if PBR or MPLS can support this. Please advice

Thanks,

Khoa

Re: OSPF load balancing

Hi,

sorry I misunderstood your setup and what you want to achieve, so MPLS TE is not solving the problem.

The only solution I know of is to do NAT with two different address pools for N on router B (10.1.1.0/24) and router C (10.2.2.0/24). This way router D can determine the return path by the destination address found in the return traffic. Whether this is worth it or even possible in your scenario I can´t tell.

Regards

Martin

Community Member

Re: OSPF load balancing

If only these four routers are in picture..

Some tme it good to use static routes and ....instead of using jargons of tech.

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