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.
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.