QOS for voice data on a Cisco 1841 router

Posted on 2007-08-01
Last Modified: 2008-12-18
Need to setup QOS on a cisco 1841 router connecting two offices with a full T1 P2P line.  I have users at remote location running citrix and just installed VOIP phone system.  Now I want to setup QOS to give priority for voice data over other network traffic.  What commands do I need to put in the routers. One cisco 1841 router at each end.
Question by:kencruzan
    LVL 6

    Expert Comment

    You may want to implement at least LLQ (low latency queueing). Just google it on

    BTW: I assume that QoS is already configured on your LAN.

    Author Comment

    No, QoS is not configured on my LAN.  I did hear that with P2P connection it is called CoS.  Is that right?
    LVL 6

    Accepted Solution

    It all depends on your P2P connection. In the case of a T1, it will not be CoS but that does not really matter.
    The default qeueuing will be FIFO and you do not want that with VoIP.
    From a QoS (generic term) prospective,I would recommend to use DiffServ end-to-end that will make your configs simpler. It all depends on your equipment and its capabilities.
    I hope that helps.
    LVL 10

    Assisted Solution

    You will need to use MQC (Modular QoS CLI) to enable LLQ (Low Latency Queuing) In order to do this:-

    Make sure the correct bandwith is specified on the interface.

    In the interface config for the T1:-
    ip nbar protocol-discovery
    service-policy voice_pq out

    class-map Voice
    match protocol rtp voice
    class-map Voice_control
    match protocol h323
    match protocol sip

    policy-map voice_pq
    class Voice
    priority percent 50
    class Voice_control
    bandwith percent 20

    What this does is:-

    1. Enable nbar protocol discovery to detect voice packets
    2. Set up class maps that categorises voice audio and control packets
    3. Set up a policy map that specifies what to doo with this traffic and how much bandwith to guarantee. (The percentages can be tweaked for your environment, but bear in mind that the priority percent percentage will determine the maximum allowed percent for voice traffic).
    4. Applies this policy map to the external interface.

    Hope this makes sense!

    Expert Comment

    sorry for piggy backing here!!!

    I'm also using 1841 and have enabled Qos to prioritize voice, however i'm experiencing choppy and delays in calls.

    via the SDM., i'm using the Qos wizard to configure QOS and assiging it to an interface.  What is the difference between using NBAR and DSCP?

    if i need to start another thread i will, thanks in advance!!

    Featured Post

    What Security Threats Are You Missing?

    Enhance your security with threat intelligence from the web. Get trending threat insights on hackers, exploits, and suspicious IP addresses delivered to your inbox with our free Cyber Daily.

    Join & Write a Comment

    Creating an OSPF network that automatically (dynamically) reroutes network traffic over other connections to prevent network downtime.
    Quality of Service (QoS) options are nearly endless when it comes to networks today. This article is merely one example of how it can be handled in a hub-n-spoke design using a 3-tier configuration.
    After creating this article (, I decided to make a video (no audio) to show you how to configure the routers and run some trace routes and pings between the 7 sites…
    Get a first impression of how PRTG looks and learn how it works.   This video is a short introduction to PRTG, as an initial overview or as a quick start for new PRTG users.

    745 members asked questions and received personalized solutions in the past 7 days.

    Join the community of 500,000 technology professionals and ask your questions.

    Join & Ask a Question

    Need Help in Real-Time?

    Connect with top rated Experts

    19 Experts available now in Live!

    Get 1:1 Help Now