On a Cisco switch, which protocol determines if an attached VoIP phone is from Cisco or from
another vendor?

A.
RTP

B.
TCP

C.
CDP

D.
UDP

Explanation:
http://computernetworkingnotes.com/cisco-devices-administration-and-configuration/ciscodiscoveryprotocol.html
The Cisco Unified IP Phone uses CDP to communicate information such as auxiliary VLAN ID,
per port power management details, and Quality of Service (QoS) configuration information
with the Cisco Catalyst switch.
Cisco Discovery Protocol (CDP) is a proprietary protocol designed by Cisco to help
administrators collect information about both locally attached and remote devices. By using
CDP,you can gather hardware and protocol information about neighbor devices, which is useful
info for troubleshooting the network.
CDP messages are generated every 60 seconds as multicast messages on each of its active
interfaces.
The information shared in a CDP packet about a Cisco device includes the following:
Name of the device configured with the hostname command
IOS software version
Hardware capabilities, such as routing, switching, and/or bridging
Hardware platform, such as 2600, 2950, or 1900
The layer-3 address(es) of the device
The interface the CDP update was generated on