Yes ,tagging is needed..
LAN QoS uses ClassOfservice(COS) bits for qos classification and marking.These bits are send along with vlan tag information.
For dot1q,its the 3 bit priority field and for isl tag its 3 bit from user field..
so for layer 2 cos, the frame must have a vlan membership that is not tagged? so am i right in saying we cannot prioritize the native vlan ?
It is possible to have a frame with a Null tag, i.e. it looks like it is on VLAN 0. It does this, just so that it can use the 3 bits for the CoS. The zero-VLAN means native, whatever that might be.
Cisco phones often use that technique. You can configure the port as an access port. The PC sends frames like it was on a normal access port. The telephone sends frames tagged with VLAN 0 and a CoS 5. These telephone frames are implicitly on the VLAN that you have configured on the port as the voice VLAN.