QoS: priority-queue out

qos

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 ?

Best Answer

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

Related Solutions

Cisco – Is it possible to shape and queue on a switched port in a cisco 4506-E

This should be possible by using Hierarchical Queueing Framework (HQF).

The parent policy-map will do the shaping and the child policy-map will do the queueing. Configuration would look something like this:

class-map CM_VOICE
 match dscp ef
!
class CM_VIDEO
 match dscp cs4
!
policy-map PM_CHILD
 class CM_VOICE
  priority
  police percent 30
 !
 class CM_VIDEO
  bandwidth percent 50
!
policy-map PM_PARENT
 class class-default
  shape average 150000000
  service-policy PM_CHILD
!

Obviously replace matching with your own values, ACLs etc.

Cisco – benefit of using CBWFQ with fair-queue statement

To summarize my comments:

The benefit of using CBWFQ (class based weighted fair queuing) with the fair-queue statement in the class-default queue (your second example):

...
 class class-default
  fair-queue
  queue-limit 1024 packets
...

Packets use WFQ (weighted fair queuing) for scheduling and are de-queued based on their calculated weight.

When using CBWFQ without the fair-queue statement in the class-default queue (your first example):

...
 class class-default
  bandwidth remaining percent 20
  random-detect dscp-based      
...

Packets are de-queued using the FIFO (first in first out) method.

Best Answer

Related Solutions

Cisco – Is it possible to shape and queue on a switched port in a cisco 4506-E

Cisco – benefit of using CBWFQ with fair-queue statement

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