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This Question is Not Answered 1 Correct Answer available (4 pts) 2 Helpful Answers available (2 pts)
5440 Views 1 Reply Latest reply: Oct 16, 2008 9:23 PM by Phosphorivera RSS

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DSCP Value Changing

Oct 16, 2008 9:23 PM

Phosphorivera 31 posts since
Sep 24, 2008

Dear QoS Expert,

 

According to the QoS Student Guide from page 2-51 until 2-59:

the "Life of High-Priority VoIP Packet" was illustrated that the DSCP from IP Phone (default set to EF = 46) is change in the output of switch to DSCP = 40. After the VoIP packet is route to the IP Phone destination, while VoIP packet arrived at the IP Phone destination, why the DSCP value is not change-back to 46? Why is still using DSCP set 40?

Thank you for your kind help.

  • Conwyn 7,914 posts since
    Sep 10, 2008
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    1. Oct 16, 2008 11:27 PM (in response to Phosphorivera)
    Re: DSCP Value Changing

     

    Hi Phosphorus

     

     

    I am guessing here but basically DSCP (IP Priority) is a layer 3 characteristic. COS is a layer 2 priority in the Ethernet frame (0-7) and since switches are layer 2 then 46 is converted to 5. On the return process 5 is converted to 40 by default. You can change this using the mls qos map cos-dscp command.

     

     

    Regards Conwyn

     

     

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