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Webcast-Catalyst9k
New Member

Variance

Is there a proper way to load-balance 2 unequal costs T1 links without using the "variance" command, assuming that all routers are running EIGRP.

Thanks.

hieu

8 REPLIES
Hall of Fame Super Gold

Re: Variance

No. And even with variance you are not assured the results you may expect, because that feature has no know implementation in any real network.

Cisco is proposing now something called OER, but it is very new, quite demanding on requisites, and doesn't seem to apply to enterprise-type networks.

Re: Variance

With EIGRP you can do equal cost or unequal cost load balancing. Variance command is used to do unequal load balancing and you don't want to use that. If you want to do equal cost load balancing the metric has to be the same. You can set the bandwidth/delay on the interfaces to be the same or use offset-list to make the metric to the same via both paths to load balance equally. If you are looking for a solution outside of EIGRP then you can use do policy routing to load balance.

HTH

Sundar

New Member

Re: Variance

Thanks for everyone responses.

Sundar: you stated that "Variance command is used to do unequal load balancing and you don't want to use that." Please help me understand of why "variance" shouldn't be used for unequal links.

Also, if you set the "bandwidth/delay" on one side of the link, does it also need to be set on the other side of the link too?

thanks.

hieu

Hall of Fame Super Gold

Re: Variance

Hieu

Each router uses the bandwidth and delay on its own interface in the calculation of the metric. It is ideal that the bandwidth and delay should match on both ends of the connection but it is not required. To the extent that there are mis-matched values it increases the probability of assymetric routing.

HTH

Rick

New Member

Re: Variance

Rick -

Thank you very much for your response. Your insight is greatly appreciated!

Thanks!

hieu

Re: Variance

Hieu,

There was a small confusion about what I said about variance. You can definitely using the variance command to load balance traffic across unequal paths. But I thought you were looking for a solution without using the variance command and that was the reason I said 'you don't want to use variance'.

EIGRP should proportionately load balance traffic across unequal paths based on the metric. If you want to load balance traffic across unequal links then configure the links to reflect the actual bandwidth and this way traffic will be load balanced according to the capacity of the link.

As far as your second question about bandwidth/delay configuration Rick has already provided a good explanation.

HTH

Sundar

Hall of Fame Super Gold

Re: Variance

Hi Sundar,

I know I repeat myself in that, but I'm still waiting (it is about 12 years now) to see a clear set of "show commands" that demonstrate that EIGRP is sharing traffic proportionally to the metric.

EDIT: I used to require the demonstration to be taken in a real production network. Since nobody even managed to do that, I would now be satisfied with a lab example, where CEF, or at worse,route-cache, is kept active and some reasonable traffic (~ 5 Kpps) is present. And to make it easier, I won't even insist on maintaining packet order !

New Member

Re: Variance

Hi Sundar,

Thank you very much for taking the time to explain things to me. I understand it now.

Greatly appreciated yours and everyone else input.

hieu

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