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3750 with nortel IP phone qos confusion

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  • 3750 with nortel IP phone qos confusion

    Hello,

    I'm new to the forum so please stick with rotten tomatoes this time out and save the rocks for later.

    I am trying to do some network changes to better accommodate our Nortel IP phone traffic in our Cisco 3750/3560 environment.

    I am attempting to apply Nortelís recommended settings for Cisco, but it seems to me that they're not making sense.

    For those not familiar with Nortel IP phones, they tag VoIP signalling with dscp 40, payload dscp 46, and CoS 6.

    This is a paste from the pertinent section of their document:
    Port based Configuration
    Config terminal (Enter global configuration mode)


    mls qos (Enable QoS globally)
    mls qos map cos-dscp 0 8 16 40 32 46 48 56 (Define ingress CoS-to-DSCP mappings)
    Intrerface level
    interface GigabitEthernet1/0/1 (Specify the physical port)
    switchport access vlan 10 (Native VLAN)
    switchport mode access (Set the port to access mode)
    switchport voice vlan20 (Voice VLAN)
    priority-queue out (Enable the egress exepedite queue)
    mls qos trust dscp (Trust IP Phone DSCP Values)
    spanning-tree portfast (For Nortel IP Phones)
    The Nortel IP Phone marks the voice payload with CoS 6 and DSCP EF when it sends the traffic
    to the switch. When the traffic enters the switch port Gi 1/0/1 (in our example), the switch trusts
    the CoS value. Then, the switch derives the DSCP value 48 for the CoS value 6 from the
    CoS−DSCP default table. Please see the output below.
    Clab_3750#sh mls qos maps cos-dscp
    Cos-dscp map:
    cos: 0 1 2 3 4 5
    6 7
    --------------------------------
    dscp: 0 8 16 26 34 46
    48 56
    We need to map CoS value of 6 to DSCP value of 0x2E or 46 (Explicit Forwarding) for Nortel IP
    Phone.
    Config terminal Enter global configuration mode.
    mls qos map cos-dscp
    0 8 16 24 32 40 46 56 (DSCP 40 for Signaling 46 for Voice and 0 for Data)
    Please note that CoS mapping is changed, CoS of 6 maps to DSCP value of 46.
    Clab_3750#sh mls qos maps cos-dscp
    Cos-dscp map:
    cos:
    0 1 2 3 4 5 6 7
    --------------------------------
    dscp:
    0 8 16 26 34 40 46 56

    You notice in the interface settings for the ingress port, their configuration tells the switch to trust the dscp tag the phone assigns to the RTP stream "mls qos trust dscp".
    However, in their subsequent description,
    "
    When the traffic enters the switch port Gi 1/0/1 (in our example), the switch trusts the CoS value"

    it says they're trusting the CoS value. Also, in their global config they have "mls qos map cos-dscp 0 8 16 24 32 40 46 56" - which says they're expecting to trust the CoS and map CoS 6 (which the phone assigns) to dscp 46. That would be reasonable if they were trusting CoS, but they're trusting dscp.

    Further, you can't trust CoS on an access port, only on a trunk port.

    Am I reading something wrong here? Or are their instructions (which have been out for a year) messed up?

    Iím confusedÖ.
    Thanks for your help in un-confusing me,
    Tarred&Fractured
    Last edited by T&F; 3rd July 2009, 15:05.
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