Which statement about Cisco Discovery Protocol is true?

Cisco Discovery Protocol (CDP) is indeed a Cisco-proprietary protocol, meaning it only runs on Cisco devices. This is a key characteristic that distinguishes it from open standards like LLDP. While CDP operates at Layer 2 (the data link layer), it does not run on the physical layer. Its purpose is to discover information about directly connected Cisco devices, including routers, switches, and other Cisco-specific hardware. However, it cannot discover information from non-Cisco devices like firewalls unless they are also Cisco-branded. CDP's functionality is limited to Layer 2, not Layer 3 (the network layer).