EIGRP Load balancing?

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I have two connections between two of our sites, a 100Mb link and a 75Mb link, all routing is done with eigrp and the 75Mb link acts as a backup.


Is there anyway to load balance between those two links with EIGRP? I know with equal cost paths it's not a problem, but since one of the lines is lower bandwidth, is there anyway to do that with EIGRP?

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Paolo Bevilacqua Wed, 02/18/2009 - 10:16
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Hi,


Your circuits have speeds similar enough for a 50% / 50%, just configure a same bandwidth for the balancing to happen.


Since the default CEF is a per-flow algorithm, you will not encounter out-of-order packet arrival despite the different speed/delay.

Jon Marshall Wed, 02/18/2009 - 10:24
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Mike


Eigrp does support unequal cost load-balancing using the "variance" command -


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml


be aware that certain experts on NetPro advise against using variance. Personally i have used it and it worked for me but it wasn't in a core part of our production network.


Jon

Paolo Bevilacqua Wed, 02/18/2009 - 10:33
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Hi Jon,


one point about using variance, it is that to my knowledge, it doesn't have feedback from the actual amount of traffic amount sent on the links. So, if statistics is on your side, and your flows have a low variance in size/duration, you will have traffic sent proportionally to link's bandwidth.


But if it not so (eg, a single, aggressive flow sticking to the slower link), you might observe that the balancing is not going well as hoped.


From this consideration, one could decide that when links are similar enough, to not bother with unequal load balancing and use them the same, as it's a stochastic game anyway.


All the above I couldn't try in practice, I think however it makes sense and will appreciate any further discussion. After all, we have it every other week :)

Jon Marshall Wed, 02/18/2009 - 12:10
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Hi Paolo


To be honest i am all in favour, if possible, of having links with the same cost naturally rather than manipulating with either variance or bandwidth/delay metrics. Obviously there is a cost to this but the simpler the better as far as i am concerned.


I agree there is no dynamic feedback as far as i can tell with variance but then the same could be said of equal cost paths. As Joseph mentioned there are the newer OER/PfR solutions which can adapt to traffic patterns altho there are issues with this too ie. just how dynamic do you want your routing protocol to be. EIGRP allows for dynamic load using the other K values but Cisco recommend against it.


Your right about it turning up every other week :-). Like i say, i have used variance in situations where having a backup link idle was not acceptable to the client and they wanted to fully utilise both links if at all possible. The first suggestion was always to upgrade the backup link if possible to ensure equal cost or to look into other alternatives such as etherchanneling or PBR. But i'm not a big fan of PBR in this scenario as it can lead to very unequal use of links.


I would be interested to hear from other engineers who have used variance in their production networks. As you say it is largely a matter of chance in load-balancing both for equal and unequal cost paths.


Jon



Joseph W. Doherty Wed, 02/18/2009 - 17:35
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"EIGRP allows for dynamic load using the other K values but Cisco recommend against it."


Does it? Yes, there's K2(?), but according to this old whitepaper, http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/ps6630/prod_presentation0900aecd80310eff.pdf, it's not dynamic. Anyone know whether this "future enhancement" was implemented?


"As Joseph mentioned there are the newer OER/PfR solutions which can adapt to traffic patterns altho there are issues with this too ie. just how dynamic do you want your routing protocol to be. "


"Long" term (minute or longer) load balancing might be one, which OER/PfR seems to handle fairly well. What you may want to avoid is injecting any load balancing route changes into a dynamic routing protocol seen by more than just the "border routers".

Joseph W. Doherty Wed, 02/18/2009 - 10:40
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"Is there anyway to load balance between those two links with EIGRP? I know with equal cost paths it's not a problem, but since one of the lines is lower bandwidth, is there anyway to do that with EIGRP?"


Yes there is, as Jon describes.


However, considering the bandwidth delta ratios, I would lean toward Paolo's suggestion and just configure for equal cost load balancing.


Another approach, if supported by your platforms, would be to use OER/PfR to dynamically load balance (unlike EIGRP which will statically load balance).

Mohamed Sobair Wed, 02/18/2009 - 11:18
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Hi,


Eigrp uses (Bandwidth+delay) as its metric of alink by default.. so dont consider only bandwdith , you should mention both, unless the Eigrp K values are configured to be calculated as its metric, although its not recommended to do so.


HTH

Mohamed

c.captari Wed, 02/18/2009 - 20:36
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I would stick with the variance. This is what cisco recommends and i find it a very good practice, as opposed to equal cost on the links. Why send equal traffic on the links if the links are not equal. Variance is very simple to configure and it would just balance the traffic proportionally according to the bandwidth/delay defined on the links.


You need to spend a bit of time and understand the metrics of EIGRP though ,if you wanna go with variance.


http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml



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