mls qos for voice vlan

1.CoS and DSCP function at different layers.Cos functions at L2,while DSCP functions at L3.
Therefore,CoS bits only remain in a subnet but DSCP value can be forwarded cross networks.

2.An IP phone is a multiport switch,with the phone set connects to a voice vlan,the pc connects to a data vlan.Thus,the connection between an IP phone and a switch is a trunk.

3.you can override the CoS value set by the PC attached to the Cisco IP phone and use the configured port priority instead. This feature is called port-based re-classification. The "switchport priority extend cos <0-7>" interface command is issued to achieve this


Here is an excellent link re QoS Q&A
http://www.cisco.com/warp/public/473/139.html