routing using ospf to distribution layer

Unanswered Question
Aug 6th, 2008

Hi all, I hope someone can clear this up for me.

Basically I have 2 x distribution switches that run the same vlans on both, I have 1 router active for all subnets, it is set to fail over to the other router if the primary fails. My question is, I want to run ospf to my core routers which there are 2 of them., so there will be 4 links in total, if I advertise the same networks on each distribution switch, ospf should load balance the traffic to and from right? if this is so, wont it try to load balance the traffic to my non active router? in this case how will it get back to the client, would it use the link in between the distribution switches for this?

cheers

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Joseph W. Doherty Wed, 08/06/2008 - 08:57

OSPF will load balance across multiple outgoing links if it sees the costs to the destination the same. From what you're describing, if all links between the routers are costed the same, upstream traffic from the one (or "active") distribution will likely go to both cores, and from both cores back to BOTH distribution routers. If traffic is returned to the "standby" distribution router, it will use the path, or paths, of least cost. This could be continuation downstream to the end hosts, via the other distribution router, or load balanced across boths. However, again assuming same link costing, traffic would likely not transit via the other distribution routers since the extra link will increase the path cost to the destination.

carl_townshend Thu, 08/07/2008 - 01:28

hi there

So are you saying some traffic going back to the dist routers may not reach the host behind the active router as it is being load balanced back to the non active router also?

can you explain if it will work and if so how ?

Joseph W. Doherty Thu, 08/07/2008 - 03:41

No, I'm not saying traffic won't return to via the "inactive" router. I'm assuming "active" and "inactive" relate to something like the HSRP gateway. I would also expect the inactive router to have connections downstream toward the host subnets. If all true, traffic should flow back to the host, but some traffic will take asymetrical paths.

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