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CEF load sharing algorithms

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Nov 3rd, 2005
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Does anyone have any information on the inner workings of the CEF load-sharing algorithms? The information on CCO is pretty scarce:


http://www.cisco.com/univercd/cc/td/doc/product/software/ios123/123cgcr/swtch_r/swi_e1.htm#wp1098076


I am especially interested in the tunnel algorithm and whether these algorithms work in per-packet or per-destination load sharing scenarios.


Thanks,

Marcel


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m.lammerse Wed, 11/09/2005 - 13:38
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Thanks for your reply.


I am familiar with the general workings of CEF.


However, I recently ran into a problem where per-packet loadsharing didn't result in an evenly-distributed load across equal cost paths.


Therefore I'm wondering how CEF can be further tuned and, specifically, how the different load-sharing algorithms work and for which environments each would be appropriate.

Richard Burts Wed, 11/09/2005 - 14:29
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Marcel


Is it possible that in the incident that you refer to the links had an equal number of packets, but that one link had a prepondernace of longer packets while the other link had a preponderance of shorter packets?


HTH


Rick

m.lammerse Wed, 11/09/2005 - 23:11
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Hi Rick,


I understand your point. There was a factor 10 difference between one link and the other on outbound traffic from the PE to the CE.


In the other direction, the load balancing was perfect.


I searched for bugs, but couldn't find anything.


The very interesting was that if we shut the lan interface on the CE (triggering an EIGRP update on the PE), the load balancing would be temporarily restored to normal. We had perfect load balancing from the PE to the CE at that point.


So it may be an EIGRP bug. But during the troubleshooting, I came across these CEF parameters which I don't fully understand. I alway thought that there was one setting to CEF : on or off.


I got the impression that the algorithms might only work in per-destination load balancing scenarios, making them useless for my case.


So my question really is, what do the different algorithms achieve and under what conditions?


Thanks,

Marcel




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