Confusion about "priority queue" and "bandwidth shape"

[michael-luo]

On LAN QoS, I've seen a configuration like below:

g0/1/0
  srr-queue bandwidth share 1 30 40 30
  srr-queue bandwidth shape 4 0 0 0
  priority-queue out

From my understanding, "priority-queue out" will make Q1 the priority  queue, which means, until Q1 is empty, there won't be any transmission  on Q2, Q3 and Q4.

If that's the case, what's the point of shaping Q1's bandwidth with  "bandwidth shape 4 0 0 0"?  Shouldn't we give Q1 100% bandwidth (since  it's priority queue)?

Thanks!

Michael

[Jon Marshall]

michael-luo wrote:
On LAN QoS, I've seen a configuration like below:
g0/1/0
  srr-queue bandwidth share 1 30 40 30
  srr-queue bandwidth shape 4 0 0 0
  priority-queue out
From my understanding, "priority-queue out" will make Q1 the priority  queue, which means, until Q1 is empty, there won't be any transmission  on Q2, Q3 and Q4.
If that's the case, what's the point of shaping Q1's bandwidth with  "bandwidth shape 4 0 0 0"?  Shouldn't we give Q1 100% bandwidth (since  it's priority queue)?
Thanks!
Michael

Michael

Which switch is this config from ?

I ask as on a 3750, for example, if you configure the priority queue then the bandwidth for that queue specified in the srr-queue bandwidth .... commands is ignored anyway. So in the above config neither the "1" nor the "4" for queue 1 are used, they are overridden as soon as the prioriry queue is enabled. This is because the priority queue is always serviced when there are packets in it so it would be meaningless to apply a bandwidth.

Obviosuly this means you must carefully plan what is allowed in the priority queue and how much bandwidth it uses otherwise it can starve the other queues.

Jon

[michael-luo]

Yes, this is a 3750 switch.

If we decided to use "priority-queue out", we should never use "srr-queue bandwidth shape" at the same time, is that correct?

Thanks!

Michael

[Jon Marshall · Accepted solution]

michael-luo wrote:
Yes, this is a 3750 switch.
If we decided to use "priority-queue out", we should never use "srr-queue bandwidth shape" at the same time, is that correct?
Thanks!
Michael

Michael

Not necessarily. The choice of using shape or shared is independant of the priority queue. When you activate the priority queue neither the shape or shared bandwidths apply to the priority queue.

Once you have activated the priority queue it is then up to you whether you choose shaping or shared mode for the rest of the queues.

Jon