ospf load balancing

yatisspor · ‎12-30-2007

Hi;

We are making a test for a project. In central site there is a Juniper M7 router, 155Mbit ATM on it. At a remote site we implemented a 2811. There are two G.SHDSL WICs on it. There are two G.SHDSL lines and lines are 2Mbits. We are trying to have a 4Mbits of bandwidth to the central site. So; we have to do some load-balancing. OSPF is used in our all WAN. So; routing protocol should be ospf. I know ospf can do load balancing when costs are the same. So i have tried it. The two ways are shown with equal metrics in "sh ip route". It seems ok. But when i look under the interfaces packets counts are not same. One is 1% of the other. How can we do this with ospf, any tricks? And how can i test it???

paolo bevilacqua · ‎12-30-2007

Hi,

the reason is because after OSPF has decided these are equal cost routes, CEF load balancing begin operating. What it does, it caches "flows" to one interface only. This is to avoid out-of-order packet arrival. Flows are, in short, calculated on the basis of IP and port source and destinations.

So if you were testing with a single flow, that is the normal behavior.

Now, if you can live with the above, in practice the only drawback is that no single flow will be able to use more than 2 mbps, but both links will be used equally.

If you are not OK with that, the alternative is to configure PPP over ATM, and them multilink PPP over said PPP links. That, if the other brand router do support such a configuration. I think the cisco does, but personally I haven't ever tested that.

Hope this helps, please rate post if it does!