QoS for VoIP on Cisco Catalyst switch

Posted on 2007-10-18
Last Modified: 2012-05-05
I need to configure QoS for VoIP on my LAN switches (Cisco Catalyst 2950, 2960, and 3560). How do I make that happen?
Question by:Kjohnsting
    LVL 3

    Expert Comment

    If it is for use with Cisco IP phones (Cisco VoIP), then I recommend you use Cisco's AutoQos. If are using Cisco phones on Cisco switches, this is made very easy.

    It is as simple as putting in the following command on each switch interface (where a IP phone is connected):
    (config-if)# auto qos voip cisco-phone

    For the interfaces which connect the switches together, you should put this command:
    (config-if)# auto qos voip trust

    This will automatically configure the best method of QoS for Cisco VoIP on the switches. The first command sets up the ports where the phones are connected to prioritize voice when competing with traffic from the PC attached to the Phone, the second command sets up the traffic between swicthes to ensure voice goes through first...... in basic terms :)

    More information here:
    Catalyst 3560 Switch Software Configuration Guide, Rel. 12.2(25)SE - Configure QoS

    The same commands applies to all of your switches, if its not available perhaps you need to upgrade your IOS's to a later version, but its been around a while so you should be ok.


    Author Comment

    We have a sister company that has an Avaya phone system. We have a private frame relay circuit between us. Some of our people use the Avaya softphone. We have a lot of dropped calls. I am assuming it is a call control issue. I am being instructed to enable QoS on my Catalysts and PIX.
    LVL 3

    Expert Comment

    Ok Avaya voice traffic uses different QoS tagging than Cisco, so that means the above Auto QoS config will not work completely. There are two ways to acheive QoS on your network for Avaya voice:

    1) Change the QoS tags the Avaya equipment puts on the traffic so that it is recognized by Cisco Auto Qos

    2) Set up the Cisco QoS to identify the Avaya traffic.

    Which one you choose really depends on your comfort level with the two technologies. Are you more comfortable with the Avaya settings or Cisco IOS configuration?

    If you choose Avaya then there a file on the PBX (46xxsettings.txt) which you can change the Qos tagging... Thats probably the easiest way to deal with it. Avaya standard Qos is COS=6 and DSCP=46 whereas Cisco uses COS=5 and DSCP=46. Here a relevant Avaya forum, there are many threads on this subject!! :

    For the Cisco option, the proceedure is manual where you can change the traffic with QoS tagging COS=5 and DSCP=46, which will then kick the Auto QoS into life.

    Regarding the PIX firewall, I beleive QoS support was only released on version 7.0 and above, so this may be a software upgrade. QoS config on a PIX is manual (no Auto QoS). Here is an article on that:
    LVL 3

    Expert Comment

    also this is a pretty difficult subject for only 125 points !!! haha

    Author Comment

    OK. If they use COS=26 and DSCP=46  on the otherside of the WAN, what is the syntax on my switches to use AutoQoS for Cisco IP. But change the default settings for Cisco phone traffic to these settings.

    And whatever that is, is that all I need to do?
    LVL 3

    Accepted Solution

    They have stated  COS=26 and DSCP=46? There is actually no such vlaue as COS=26, was this meant to be COS=6?? If so thats the standard for Avaya traffic. What I strongly suggest is you ask them to change there Avaya QoS settings to COS=5 and DSCP=46, as mentioned this just requires changing a text file (46xxsettings.txt). Then you just run autoQoS as the commands above. The reason I suggest this method is because modifying the QoS on the switches is complicated business and if done incorrectly it can majorly impact your network.

    And actually the PIX is most likely to be your "bottleneck". If there is voice quality issues caused by your equipment, then it will be at pix/internet connection, it is VERY uncommon to see a Cisco switch dropping voice packets or causing jitter etc. Most ISP's do not support QoS on there WAN links also.

    However your question was how to configure QoS on Cisco switches. Have we covered that?

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