which protocol determines if an attached VoIP phone is from Cisco or from another vendor?

Cisco Discovery Protocol (CDP) is very crucial in the operation of a Cisco IP phone. It not only provides the AUX (Voice) VLAN ID for the phone to being sending traffic on the AUX VLAN, it also allows the phone to automatically negotiate power settings. This allows the phone to use less than the maximum 15.4 watts of power under the 802.3af PoE standard. If you disable CDP on the port facing the phone/PC you will likely start pulling your hair out. Even though the phone might have already assigned itself in the Voice VLAN, removing CDP from the switchport in question causes it to forget where to find the voice VLAN. You’ll need to re-enable CDP and reboot the phone.