WAN load balacing over congested links

Unanswered Question
Sep 21st, 2007

Hello: Does the cisco routers perform smart load balancing? if so, how does the router know, which path is congested than others.

Thank you

I have this problem too.
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spremkumar Fri, 09/21/2007 - 04:58

hi narayan

AFAIK routers normally forward the traffic via the alternate interface based on the backup interface / backup load command you configure under the main forwarding interface.

without doing so if you have 2 static routes pointing towards the active interfaces it may result in either per-packet or per-session based load balancing based on the type you prefer and configure out there in the box.

regds

paul.matthews Fri, 09/21/2007 - 05:16

EIGRP has a feature that will allow the load of an interface to be included in the metric, and a feature to allow unequal cost load balancing, but both of these are off as default.

To turn on load use:

router eigrp

metric weights 0 1 1 1 0 0

Default would be

metric weights 0 1 0 1 0 0

That would then include the load in the metric calculation.

To allow uneven cost load balancing:

router eigrp

variance 2

will allow the high meteric used to be dounle the low metric.

Having said this, having interfaces with effectively variable metrics where the metric can influence the metric (I'll come back to that) may not be the most stable idea.

As I say about metric influencing metric just consider this.

You have simply a pair or routers, with a pair of matching links between them, running eigrp across the links. Before there is any traffic, the metric for both is equal. Due to the nature of the traffic, one link gets a little more traffic, so eigrp adjusts the metric to favour the less loaded link. Traffic not starts to favour the previously light link, so that now becomes the heavier used link, so EIGRP again adjusts the metrics...

ksnarayan43 Fri, 09/21/2007 - 07:18

Thank you . Do other routing protocols support this kind of feature.

thank you

paul.matthews Sun, 09/23/2007 - 23:28

I am not aware of any, and as I meantioned before, the whole idea is rather risky. If stability is improtant in a network, I wuld strongly advise against the use of such features!

Paul.

ohassairi Mon, 09/24/2007 - 03:42

paul

if the 2 connections were PPP, does PPP multilink resolve the problem automatically?

paul.matthews Mon, 09/24/2007 - 04:31

That's a different question

PPP multilink will distribute the load - off hand I am not sure exactly how it does it - round robin, first available path, some other algorithm.

If it was a pair (or more) of links between two routers MLPPP would probably do the job, if you are looking for intelligent re-routing around a ;arge network based on traffic levels, then it would be less useful.

Once you start considering multi chassis, it wil almost certainly be counterproductive.

ksnarayan43 Fri, 09/21/2007 - 07:38

Thank you . Do other routing protocols support this kind of feature.

thank you

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