[c-nsp] etherchannel load-balancing and unpredictability

[John Gill]

Hello Steve,
The port selection function is based on a hash of the inputs, in this 
case the source and destination IP address, and the output is a value 
that chooses a member interface.

These functions between the input and output vary by platform, but for a 
given platform you can expect with the same flow, and even the reverse 
of that flow, you will get the same selection.

Because the platforms implement this selection logic in hardware, they 
can and *do* vary between one model and another.  As a matter of fact, 
the new Nexus 5500 has the ability to choose between various polynomials 
used in this function.  You might find one gives you a better 
distribution over another.  In your case, it sounds like you need some 
state on both sides of the content filter to be in tact.  In this 
scenario, you would need the same platform on both sides to guarantee 
this kind of behavior.

Regards,
John Gill
cisco


On 7/19/11 4:11 PM, Steven Pfister wrote:
> I have a question regarding etherchannel load balancing. I've got a
> 4507R switch connected to a 3560 switch by means of two content filters
> which are acting as transparent bridges. The two ports on each side that
> the content filters are connected to are set up as access ports and are
> in an etherchannel. The load balancing method on each switch is set to
> src-dst-ip. I was under the impression that each pair of source and
> destination ip address would select exactly one content filter no matter
> which direction.
>
> I've been told that this can be 'unpredictable' and may cause
> assymetric flows. The algorithm seems fairly straightforward to me. I
> don't see where the unpredictability can come in. Can someone explain to
> me what I'm missing?
>
>
> Steve Pfister
> Network Engineer
> Office of Information Technology
> Dayton Public Schools
> 115 S Ludlow St
> Dayton, OH 45402-1812
> Phone: 937-542-3149
> Cell: 937-673-6779
> spfister at dps.k12.oh.us ( mailto:spfister at dps.k12.oh.us )
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