Priority queue out handling.

Unanswered Question
Mar 7th, 2012
User Badges:

We're testing the reference system shown in the figure below.

System Description

  • Four 2960 switches are used for transport;
  • Equipment 1 and Equipment 2 exchange packets for synchronization;
  • To reach synchronization  Equipment 1 and 2 must exchange data with a very low jitter.


2960 Configuration details

Four our test puprose, we're using 100Mbit/s ports (22 and 23) as trunk.

In order to obtain minimum jitter We performed these configurations:

  1. We Enabled QoS;
  2. We Marked Synchronization packets with CoS 7 and DSCP 63;
  3. We marked other kind of traffic inserted in different ports) with CoS 0;
  4. We set "trust DSCP" on trunk ports;
  5. On the trunk ports we mapped traffic with CoS 7/DSCP 63 (and only this) on output queue 1;
  6. We enabled the expedite queue (priority-queue out).


With these settings we aim at forcing our synchronization packtes to precede other kind of traffic and go from Equipment 1 to Equipment 2 with minimum jitter.

Unfortunately we experienced  high jitter when both synchronization packets and other traffic are sent through the systems.

What is wrong in our assumptions or configurations?

What is the real behaviour of the expedit queue?

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (0 ratings)
Daniele Giordano Wed, 03/07/2012 - 10:00
User Badges:
  • Gold, 750 points or more

Seems the right config.

This is the my configuration and it works well:

#global level

mls qos srr-queue input priority-queue 2 bandwidth 20

mls qos srr-queue input cos-map queue 1 threshold 3  0 1 2 3 4

mls qos srr-queue input cos-map queue 2 threshold 3  5 6 7

mls qos srr-queue output cos-map queue 1 threshold 3  5 6 7

mls qos srr-queue output cos-map queue 2 threshold 3  4

mls qos srr-queue output cos-map queue 3 threshold 3  2 3

mls qos srr-queue output cos-map queue 4 threshold 3  0 1

mls qos

#port level

mls qos trust dscp

priority-queue out

What is your IOS version?

Can you use the 2960 Gigabit port?


massimosanto Thu, 03/08/2012 - 05:27
User Badges:

Ciao, Daniele

Thank You for your answer.

Our Version is 12.2.

We use 100 Mbit/s ports because for our test purpose we need to saturate a link and we don't have a Gigabit capable traffic generator.

But what is different with a Gigabit port? We think that, since expedite queue is enabled, our synchronization packets should be preferred both in the case of a 100Mbit/s and in the case of a Gigabit port.



massimosanto Fri, 03/09/2012 - 04:49
User Badges:

Thank You, Daniele

That's the guide we used to create the configuration described in my first message.

But our tests seem to reveal a behaviour of the expedite queue from the one described in the guide.

We will install IOS 15 and repeat the tests.


massimosanto Wed, 03/14/2012 - 06:28
User Badges:

We tested also the new IOS and nothing is different.

In our opinion, priority queue out command ensure us all the bandwith we require but it seems to not guarantee us minimum jitter.


Agustin J Sanchez Sun, 03/18/2012 - 05:52
User Badges:

I am reading on the web because I want to achieve nearly the same as you, Massimo. I am confused about the following: is it expedite queue (priority-queue) the output queue 4 instead of 1? You are sending your Synchronization packets to queue 1, but maybe you have to send them to queue 4?

massimosanto Mon, 03/19/2012 - 01:13
User Badges:

Hi, Agustin

I'm pretty sure the priority queue is queue #1.

I verify this condition by applying a shaping on queue#1, inserting a traffic load exceeding shaping limit and then enabling priority queue.

In this situation shaping is ignored and all the traffic sent through queue #1 is allowed to flow.

In any case I tried also to send packet through queue#4.

Thank You,


massimosanto Fri, 04/27/2012 - 03:22
User Badges:

Hi, Agustin

No other news about Cisco configuration.

I believe that the issue we found is just a limit and cannot be overcome.

I think expedite queue can guarantee the bandwith but not the minimum delay.

Sorry for the delay in my answer.



This Discussion

Related Content