Howto configure output traffic shaping on Cat3560E switch?

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Jan 25th, 2010
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Hi,

I want to shape traffic outgoing a 3560E switchport. The port speed is 100Mb full duplex but the bandwith we can use on the connected wan link is only 30 Mb/s.


I already checked the configuration guide and found the shaped round robin (srr) commands (http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_50_se/configuration/guide/swqos.html#wp1200681).


Do I understand it right that I need to configure the "srr-queue bandwidth limit <weight>" command (http://www.cisco.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_50_se/command/reference/cli3.html#wp1947391) to shape the bandwith to 30mb/s?

Does anybody know what is the base of the weight percentage value, is it the physical interface speed, the configured interface speed (or autonegotiated value) or the bandwith parameter I can configure on the interface?


Many thanks in advance,

Thorsten

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Giuseppe Larosa Mon, 01/25/2010 - 11:37
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Hello Thorsten,

use of srr-queue bandwidth limit has been reported by other colleagues in the forums.


This is a form of shaping. However, who used these commands reported a behaviour different from shaping on a router.

That is control is less precise, limiting is effective but it appears more limiting then expected.


see for example


https://supportforums.cisco.com/message/581759#581759


Usage Guidelines


If you configure this command to 80 percent, the port is idle 20 percent of the time. The line rate drops to 80 percent of the connected speed. These values are not exact because the hardware adjusts the line rate in increments of six.


my understanding of connected speed is of actual speed of the link and so it was in the that other thread a limiting to 400 Mbps had been reported using 40.


Then you need or to configure all queues for shaped SRR mode with the other command you have mentioned or to map all possible CoS values to one or two queues that use shaped SRR. For emulating shaping the first approach is enough.


srr-queue bandwidth shape 10 10 10 10

+

srr-queue bandwidth limit 30


should be enough in your case.


Hope to help

Giuseppe

TodirascuOvidiu Wed, 02/17/2010 - 15:11
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Hello,


I belive this configuration is not correct. The formula used to assign a certain bandwidth to queues when shaping on egress on 3560 is portspeed x 1/weight

So in this config for example queue 1 would get 100Mbps x 1/10 = 10Mbps etc

Also a weight of 0 means that queue operates in shared mode.


http://www.ciscosystems.com/en/US/docs/switches/lan/catalyst3560/software/release/12.2_25_se/configuration/guide/swqos.html#wp1163879


srr-queue bandwidth limit - rate-limits the interface bandwidth


Hope this helps



thorsten.steffen Thu, 02/18/2010 - 07:12
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Hello,

I tested with "srr-queue bandwidth limit 30" on a fast ethernet full duplex switchport, one fluke etherscope traffic generator connected to this port, a second one connected to another switchport.

The troughput measured ranges between 40Mb/s @ 64b frame size and 30,5Mb/s @ 1518b frame size.


So shaping generally works but not exactly.

Does anybody have an idea what's the reason?

TodirascuOvidiu Thu, 02/18/2010 - 08:46
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Hello,


As per my understanding limiting on egress on a 3560 using  "srr-queue bandwidth limit 30" drops the excess and does not delay it as you asked in the first post.

Regarding the test results are you sure they are accurate ? Have you tried testing this with other tools/real traffic ?


Hope this helps.

thorsten.steffen Fri, 02/19/2010 - 00:32
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Hi,

in my eyes this limiting shapes small packets and starts dropping packets bigger sizes. This would explain why small packets get through with 40Mb/s and big packets only with 30 Mb/s.

Today I sniffed the traffic (generated by the fluke) at the outgoing switchport (which is limited to 30%). In the trace I saw a constant throughput of 30Mb/s while fluke showed the same results I mentioned above. So it seems that the switch is shaping (delaying) small packets and dropping big packets.

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