Cos and cisco ip phone trunking

[meraghnis]

What is the default behaviour of a cisco ip phone regardless of trunking. Does it use dot1q encapsulation and sets Cos to 5.

In case we use the command (config-if)#switchport voice vlan untagged on the switch, what effect does this command has on the encapsulation of voice on the ip phone. Does it also affect the Cos setting of the ip phone.

[pbarman]

If you do not specify any trunking config (default - switchport voice vlan none) on the switch port, by default the phone will not do any trunking, and all traffic from phone will be untagged 802.1q/802.1p frames with CoS 5. In this case COS value of IP Phone is dictated by Callmanager Service Parameter, and by default it is 5. Any CoS value configured on the switch port is ignored.

When you use the "switchport voice vlan untagged" command, the IP phone sends untagged 802.1q voice traffic like above. However the CoS value can be overridden by configuring a value on the switch port.

[meraghnis]

I am still confused by the fact that traffic from phone can be untagged and at the same time CoS set to 5. how can this be done sine we know that an untagged frame doesn't carry 802.1Q/802.1P information.