OSPF unequal cost load balancing

[jacquesd]

Hi,

Is there any way to load-balance via OSPF with a cost difference as small as 2 FastEthernet links (Cost difference of 2)? I need to decide on EIGRP (for which variance will solve the problem, but will cost some money) or get OSPF to work over unequal cost links.

Thanks!

Jacques

[smif101]

Under the interface you can use the ip ospf cost # command to set the two interfaces the same and then the router will load balance between them.

[jacquesd]

Hi,

I have considdered that, but will have no effect:

Frame-Relay

* *

*100 *100

RTA RTB

* *

*1 1 *1

HostA***SWA*******SWB***HostB

(I hope my drawing comes out right when posting!)

Where RTA and RTB is the routers and SWA and SWB is

the switches and the costs indicated.

The cost for SWA to reach RTB is 2 and to reach RTA is 1. If the costs on one side is changed, the other side will not load-balance.

Thanks

Jacques

[jacquesd]

OK, obviously not...

The thing is, like I mentioned, adjusting the costs for one side will stop the other side from load-balancing. So I cannot get away from the fact that the links will be of unequal cost.

Thanks!

Jacques

[smif101]

Yeah I don't know what to tell you, I really don't understand your drawing but there is always a way to get everything to be equal cost. If you have a switch to switch issue than you can enable etherchannel to help you there.

[vcjones · Accepted solution]

First let me make sure I have your topology right: HostA connects to SwitchA (which is actually a router) which connects to both Switch B and RouterA. Router A then connects to RouterB and via frame relay to somewhere remote. Switch B connects to HostB and RouterB, and Router B connects to the other frame link.

As you have noted, it is very difficult and often impossible, to tweak link weights so that all routes are equal cost (you don't mention, but also need to worry about return traffic being load balanced, because the unbalance will be reflected in the routes leading to the frame relay network on the other side).

While EIGRP variance might get you closer, if you look closely at how it works, you probably won't like its shortcomings. In particular, EIGRP unequal path load balancing only works between paths which are feasible successors, which your current routes, as currently weighted, will NOT be.

The standard solution to provide load balancing in this scenario is to add cross links from SwitchA to RouterB and from SwitchB to RouterA. That way, there really are equal cost paths available for load balancing and you can take full advantage of OSPF's handling of parallel routes.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com

[jacquesd]

Hi,

Yes, you understand the drawing (sorry about that), and I understand what you are saying about crossing the links. That will work perfectly, I did not think of that. Thanks!

What will have to happen is that the remote sites (this is HQ, hub and spoke) will have to have PVCs mapped to both routers A and B for return traffic to work. That was agreed to previously.

But what I heared today is that they are considdering switching to a MPLS network for various (valid) reasons. I spoke to the SP and they can only provide BGP connectivity, I am not too familiar with MPLS, so I am not sure how and if load-balancing is still an option. The reason for the two links (they are in separate buildings) is for Disaster Recovery purposes.

Any suggestions?

Thanks again

Jacques

[vcjones]

Using BGP & MPLS, load balancing is still an option, but routing in general is more complex because your WAN systems are no longer "adjacent" so OSPF will not work over the WAN unless you use GRE tunnels (which you hopefully will not want to do). Instead, you use BGP between your access routers and the service provider, and redistribute the routes (which are your other networks and routers on the far side of the MPLS network) into your local OSPF routing protocol.

Load sharing works fine, provided you understand what you are doing when you assign weights to the routes when you redistribute them. Its really not that difficult, but you do have to pay strict attention and take the time to understand what you are doing so that you don't introduce routing loops or other problems.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com