VoIP - Voice VLAN
A voice VLAN is a specialized VLAN on a switch that is used to transmit voice over IP frames. It can be configured in such a way so that the switchport on which it is configured can have an IP phone connected to it, and a PC connected to the phone. The voice will travel on the voice VLAN while the data from the PC will be placed on the data VLAN.
An IP phone will learn about the voice VLAN from Cisco Discovery Protocol (CDP) messages.
CDP will inform the IP phone of the voice VLAN ID being used and will also let it know about the CoS values the phone should configure for its voice traffic. If you disable CDP, then you must manually configure the voice VLAN on the IP phone. It will still work, but you need to take that extra step of manual configuration on the IP phone.
A voice VLAN configuration is achieved by using the voice vlan X command, where X denotes the voice VLAN ID. Consequently, on this port, only frames that belong to VLAN X will be tagged when transmitted. Similarly, the connected phone will also send tagged traffic solely on VLAN X, while any untagged traffic will be sent on the VLAN the access port is configured on.
If you’re using a non-Cisco phone that doesn’t speak CDP, then you will have to configure the voice VLAN on the phone anyway.