Command"mls qos trust cos" and"switchport priority extend cos" use together

[teru-lei]

Hi all,

When config a 3550 port with voice vlan, it's recommand that command "mls qos" in config mode and "mls qos trust cos" in interface mode be used. But when I want to reset the cos of the data traffic, command "switchport priority extend cos" should be used. So if I use "mls qos trust cos" and "switchport priority extend cos" and the same time, it make me confused, will the port trust the cos of the data traffic or override it? Can anybody tell me the truth? And is there any command to verify it? Thank You!

Best Regards

Teru Lei

[b.speltz · Accepted solution]

With both the "mls qos trust cos" and "switchport priority extend cos" together, the switch trusts the CoS values in the packets from IP Phones, but it overrides the CoS value in data frames from any attached PC with the value configured on the switch. You can go to this link for more info though it is not related to 3550 :

http://www.cisco.com/warp/public/473/139.html