QoS: priority-queue out

Question

I'm trying to understand the priority-queue out command configured on an interface.

Based on what I have read, here is what it does : the Priority Queue (Queue 1) on the switch will be emptied before any other queues (Q2, Q3, Q4).

But how can you tell which traffic should be sent to Q1 ? Is this done using a class-map and a policy-map ? Based on this document : https://www.cisco.com/c/en/us/td/docs/routers/connectedgrid/cgr1000/1_0/software/configuration/guide/qos/cgr1000_Book/qos_priority_cgr1000.pdf ... you would have to configure "priority level 1" in your policy-map. Is this correct ?

We have a switch here configured with priority-queue out, but I can't find any class-map or policy-map on it. Is it possible to assign traffic to the priority queue in another way ?

Answer 1

Actually, I just found the answer.

You can assign CoS/DSCP values to specific queues. There are 4 queues for egress traffic. Since Q1 is the priority-queue, here is an example that would assign CoS value 7 to Q1 Threshold 2:

mls qos srr-queue output cos-map queue 1 threshold 2 7

Here is the same for DSCP value 56 :

mls qos srr-queue output dscp-map queue 1 threshold 2 56

Traffic marked with CoS value 7 or DSCP value 56 would be sent to Q1, and thus the command priority-queue out should empty this queue before anything else, and prioritize our traffic.

Please note that this is for egress traffic. Ingress traffic works differently, since it has only 2 queues (Q1 and Q2), and Q2 is the priority queue by default.

Each queue has multiple threshold (3 in total per queue) which can also be configured, but there are default values if you don't. Thresholds values define when traffic starts to get dropped. There is a good explanation on thresholds here : https://community.cisco.com/t5/telepresence-and-video/help-understanding-qos-threshold/td-p/1374101

Answer 2

The 2960 switch uses the mls qos commands to assign QoS.

You can use access lists to identify different traffic and classify it with class maps to assign different traffic to different priorities, and the mls qos commands to assign the queues.

The document you are looking at is specific for a router, not a switch. You need to look at a document that is for your switch, e.g. Catalyst 2960-X Switch QoS Configuration Guide, Cisco IOS Release 15.0(2)EX.

The entire subject is too large to discuss here, but you can ask other questions for specific problems you may have.