srr queue - shape and sharing

Unanswered Question
Jan 26th, 2010

Hi all

can anyone tell me what the sharing option is for when using the srr queue command?

i see there are weights you can assign, what is this?

cheers

I have this problem too.
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sachinraja Tue, 01/26/2010 - 07:10

Hi Carl

This is the average ratio of packets which could flow in each egress queue. in switches, you have 4 egress queue, and there should be some mechanism to rate traaffic flowing through these 4 queues.. by specifying weight, we make sure each queue has a ratio of percentage of traffic flowing through it.. as given in CCO,

example:

srr-queue bandwidth share 1 2 3 4

first queue has 10 %, second has 20 %, 3rd has 30 % and 4th has 40 % of the overall bandwidth allocated. percentages are calculated as normal - 1/(1+2+3+4) which is 1/10 - 10 % , similarly for other queues.. what are you trying to accomplish here ? Its sometimes really complicating to do port level qos or bandwidth allocation on edge switches. granular qos is normall done on the core, with edge just doing qos marking...

Hope this helps..all the best

Raj

Ganesh Hariharan Tue, 01/26/2010 - 07:16

Hi all

can anyone tell me what the sharing option is for when using the srr queue command?

i see there are weights you can assign, what is this?

cheers

Hi,

srr-queue bandwidth shape interface configuration command to assign the shaped weights and to enable bandwidth shaping on the four egress queues mapped to a port.Specify the weights to specify the percentage of the port that is shaped. The inverse ratio (1/weight) specifies the shaping bandwidth for this queue. Separate each value with a space. The range is 0 to 65535.

In shaped mode, the queues are guaranteed a percentage of the bandwidth, and they are rate-limited to that amount. Shaped traffic does not use more than the allocated bandwidth even if the link is idle. Use shaping to smooth bursty traffic or to provide a smoother output over time.

Hope that help

Ganesh.H

carl_townshend Tue, 01/26/2010 - 08:53

Hi There

I am ok with the shaping, its more the sharing bit, what benifit would you have in the sharing mode? what does it acheive by sharing?

cheers

sachinraja Tue, 01/26/2010 - 08:54

Hi carl

Can you check my previous post ? Does it answer your query ?

Raj

carl_townshend Tue, 01/26/2010 - 11:46

hi there

what I really want to know, it what does the sharing do?? i know shaping smooths the flow to a certain bandwidth and buffers it outbound to do this, correct?

it is the sharing one i want to know more about, ie why would you need to do this in the first place?

cheers

Jon Marshall Tue, 01/26/2010 - 11:55

carl_townshend wrote:

hi there

what I really want to know, it what does the sharing do?? i know shaping smooths the flow to a certain bandwidth and buffers it outbound to do this, correct?

it is the sharing one i want to know more about, ie why would you need to do this in the first place?

cheers

Carl

i know shaping smooths the flow to a certain bandwidth and buffers it outbound to do this, correct?

Correct on a router but a L3 switch doesn't really support shaping, with the exception of some of the WAN interfaces available on the 6500.

Basically on a L3 switch -

shaping - if the queues are shaped then they are guaranteed the bandwidth configured but you cannot go above the bandwidth configured on the queue ie. they are rate-limited and even if there is spare capacity in another queue it cannot be used

shared - the queues are again guaranteed the bandwidth configured but if there is spare capacity in another queue it can be used. The weights are used to determine how much of the extra capacity each queue can use.

All of this is explained in the QOS config for most catalyst switches, have a look at this section which covers your exact question -

SRR Shaping and Sharing

Jon

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