Voice (VoIP) VLAN Tagging

The VoIP phone is a three port switch:

The switch will use CDP to inform the phone of the Voice VLAN
The phone will tag its own traffic (Voice) and can set Cost of Service (CoS). CoS is QoS for layer 2

Traffic	Tagging Function
Voice VLAN	tagged with a Layer 2 class of service (CoS) priority value
Access VLAN	can also be tagged with a Layer 2 CoS priority value
Access VLAN	is not tagged (no Layer 2 CoS priority value)

CDP – Cisco Discovery Protocol

The phone may or may not tag frames from the PC