Which Cisco Unified Communications Manager protocol communicates with collaboration endpoints?

SCCP (Skinny Client Control Protocol) is a legacy Cisco-proprietary protocol used with older IP phones that is being deprecated in favor of SIP and does not represent the current standard for CUCM collaboration endpoint communication.

RTP (Real-time Transport Protocol) carries the voice and video media stream between endpoints after a call is established but is not a signaling or registration protocol managed by CUCM for endpoint communication.

SIP (Session Initiation Protocol, RFC 3261) is the industry-standard and Cisco-preferred protocol for endpoint registration, call setup, and teardown in modern CUCM deployments. It is supported by all current Cisco collaboration endpoints and third-party devices, making it the primary protocol CUCM uses to communicate with collaboration endpoints.

CDP (Cisco Discovery Protocol) operates at Layer 2 to discover neighboring Cisco devices and exchange information such as VLAN IDs, performing no call signaling or endpoint registration function within CUCM.