2950 QoS for voip traffic

I was poking around cisco.com and found an FAQ on qos features supported by 2950s and I was confused by the last line of the following answer.

In Cisco IOS Software Release 12.1(12c)EA1 and later, there is support for expedite queue and WRR scheduling instead. This scheduling uses one of the four egress queues (queue 4) as an expedite queue. WRR scheduling is performed on the three egress queues that remain. The expedite queue is a strict priority queue. Before the service of the other three queues, this expedite queue always receives service first, until it is empty. For all releases, strict priority scheduling is the default. Here is a sample configuration:

wrr-queue cos-map 1 0 1 2 4
wrr-queue cos-map 3 3 6 7
wrr-queue cos-map 4 5
wrr-queue bandwidth 20 1 80 0

Note: CoS 5 is mapped to queue 4. Queue 4 is the expedite queue, with bandwidth assigned to 0.

why would you want to set the bandwidth of the strict queue to 0? This confused me so I went over my notes from my BCMSN text book and I wrote that you have to configure the lower queue with lower weights. Is there some other pricipal to wrr queueing that I am not aware of?

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Comments

lildeezul

it doesnt matter what bandwidth you assign to the priorty queue, its always going to get priority.

fonque

does setting the queue 4's bandwidth to 0 turn on priority queuing for a 2950?

Mrock4

Yes..it does. Configuring Queue 4's bandwidth as 0 enables strict-priority queueing for Queue 4. It will service Queue 4 first, until it is empty, then use WRR for the remaining 3 queue's.

Don't think of it in too strict of terms- bandwidth being "zero" for Queue 4 just tells it that it is functioning as the priority queue, whereas on a 3550 you could use "priority queue out"

lildeezul

Yeah , its specific the the catalyst switch model.

with the 2950, you have to use 0, with the 3550, it doesnt matter what number you put in there.. i believe

Mrock4

Plus, FWIW, AutoQoS automatically sets the bandwidth value for Queue 4 at 0.

And you're correct, on a 3550 with priority-queue out, it will ignore the 4th weight.

fonque

Mrock4 wrote:

Yes..it does. Configuring Queue 4's bandwidth as 0 enables strict-priority queueing for Queue 4. It will service Queue 4 first, until it is empty, then use WRR for the remaining 3 queue's.

Don't think of it in too strict of terms- bandwidth being "zero" for Queue 4 just tells it that it is functioning as the priority queue, whereas on a 3550 you could use "priority queue out"

thats exactly what I was looking for. Thank you!