3750 StackWise limitations

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Sep 10th, 2010
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Hi All,

I have a question regarding the limits of the standard StackWise technology (not StackWise+).

I have 5 x WS-3750G-24-TS in a stack connected at full bandwidth with the stacking cables.

If I read correctly then every frame that passes through the switch stack has to hit the Stack-Ring due to the lack of Local Switching with the StackWise technology, so if I'm correct does that effectivley mean that the maximum per port bandwidth can be figured out by the forumulae:

Stack Ring Bandwidth / Number of ports connected at Full Duplex.

So in my case:

32Gbps / 140 ports @ 1Gbps.

If the above is correct is this a hard limit as in the port would only function at around 228Mbps even when not busy or would only burst to a maximum of 228Mbps as dictated by the Stack-Ring bandwidth?

Many Thanks,

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francisco_1 Fri, 09/10/2010 - 09:10
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I found your post to be very interesting.

Regarding the local switching, the StackWise plus can locally switch; Stack Wise cannot.  In Stack wise pls, packets originating and destined to ports on the same local switch will not have to traverse the stack ring, which results in more efficient switching.

In contrast to stackwise, because there is no local switching and because there is source stripping, even locally destined packets must traverse the nrtire stack ring.

Hope that helps.


DuncanM2008 Fri, 09/10/2010 - 09:36
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Hi Francisco,

Thanks for your post

Basically that's kind of what I'd already come to the conclusion of about StackWise in comparison to StackWise Plus.

I'm running normal StackWise not StackWise Plus/+  thus all my traffic will be hitting the Stack-Ring.

Which logically would suggest that the 140 ports connected at 1Gbps full-duplex can reasonably only expect to be able to actually transmit/recieve at around 228Mbps based on the forumulae (32Gbps divided by 140 (Number of ports).

The more intersting question if the above is true is;

Is the 228Mbps limit burstable as in it's only a hard-limit when the full stack bandwidth is being consumed or is it a pre-calculated limit based on the number of ports divided by the stack-ring bandwidth and as such will always be throttled regardless of how busy the stack-ring is?

Thanks in advance,

francisco_1 Fri, 09/10/2010 - 10:06
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I think based on your calculation on a 24 ports switch version, where the ports not operating at full wirespeed at the same time switch, you might get away with that calculation. The 48-port versions will not (48Gbps all ports vs 32Gbps backplane).

Have you looked at the Cisco 6500 series can have switching backplanes as high as 720 Gbps



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