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List: cisco-nsp
Subject: Re: [c-nsp] FW: Overruns
From: Nick Hilliard <nick () foobar ! org>
Date: 2011-01-27 16:36:15
Message-ID: 4D419EFF.7070102 () foobar ! org
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On 27/01/2011 16:00, Brett Frankenberger wrote:
> Of course he is.
It certainly adds extra resilience if you have an individual bearer link
failure. But it also not unreasonable to assume that adding links into a
LAG will increase its throughput. Unfortunately, this is often not the
case on lots of Cisco hardware - including sup720 based equipment.
Taking your example below, if you had an 8 port LAG and one of them failed,
this would drop you down to a 4 port LAG in terms of throughput. If you
aren't expecting it, this is astonishing behaviour and have pretty serious
consequences.
This wouldn't be so much of a problem if the restriction were well
documented by Cisco. Unfortunately, it's not - you have to dig to find it,
and then infer that the same restrictions in CatOS also apply to IOS. I
guess it's not something that Cisco like to make too much noise about.
Nick
> With five links, assuming the traffic hashes evenly
> across the 8 buckets, he effectvly has 4GBps of throughput available.
> If one of the five links fails, he still has 4GBps of throughput
> available.
>
> With four links, assuming the traffic hashes evenly across the 8
> buckets, he effectively has 4Gbps of throughput available. But if one
> of the four links fails, then he'll hash at 3:3:2 and effectively have
> 2.67Gbps available.
>
> In other words, the fifth link doesn't add any throughput benefit in
> the "everything is working" case -- four or five active links offers
> the same throughput -- but it offers a significant redundancy benefit.
> With five links, loss of one link is no impact to cpacity; with four,
> loss of one link is a 33% reduction in capacity.
>
> -- Brett
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