Ask the Experts - Campus QoS

Unanswered Question
May 28th, 2010

Hi Hatim,


I have a question about QoS on the Catalyst 3560 switch.  When the priority queue is in "shaped mode", is the assigned bandwidth limited to the configured amount or can the priority queue consume more than it's allocated bandwidth.


For example, I know the "srr-queue bandwidth shape 3 0 0 0" command puts Queue 1, the priority queue, in shaped mode and that it is guaranteed one-third of the interface's bandwidth.  Is the priority queue limited to just one-third or can it consume more than this?


Cisco's documentation is sketchy on this and I have found conflicting descriptions elsewhere on how this works.


Thanks in advance for your response.


Ian.

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Jon Marshall Fri, 05/28/2010 - 09:21

IHCowan wrote:


Hi Hatim,


I have a question about QoS on the Catalyst 3560 switch.  When the priority queue is in "shaped mode", is the assigned bandwidth limited to the configured amount or can the priority queue consume more than it's allocated bandwidth.


For example, I know the "srr-queue bandwidth shape 3 0 0 0" command puts Queue 1, the priority queue, in shaped mode and that it is guaranteed one-third of the interface's bandwidth.  Is the priority queue limited to just one-third or can it consume more than this?


Cisco's documentation is sketchy on this and I have found conflicting descriptions elsewhere on how this works.


Thanks in advance for your response.


Ian.


Ian


Shaped means the queue is guaranteed the configured bandwidth and is rate-limited to that amount. So even if there is spare bandwidth it cannot be used if it goes above the configured bandwidth.


By the way i think you meant to post this into the Ask the Experts thread ?


Jon

Jon Marshall Fri, 05/28/2010 - 14:16

IHCowan wrote:


Hi Jon,


Have a look at the answer to the 3rd Question in the Quick Questions and Answers section of this article.  The author seems convinced that the priority queue can consume more than it's allocated amount and later in the Q&A mentions he has even tested this to be true.  Ian.


http://blog.ine.com/2008/06/26/quick-notes-on-the-3560-egress-queuing/


Ian


You are right and the key point i missed was that you were referring to the priority-queue. When you configure a priority queue it overrides the shaped or shared weights and it is indeed serviced until it is empty.


So shaped and shared mode apply only to the non priority queues.


Jon

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