Cisco Discovery Protocol is a protocol for discovering devices on a network. Each CDP-compatible device sends periodic messages to a well-known multicast address. Devices discover each other by listening at that address. CDP has primarily two features, Advertisement and Discovery. The CDP implementation for Unity addresses only the advertisement feature. CDP advertisement communicates the system name, interface name, system capabilities, etc. through sending periodic CDP messages on the active physical interfaces (Ethernet NIC) to a well-known multicast address (0100.0CCC.CCCC). CDP discovery allows the device to listen to the periodic advertisement messages sent by other neighbor devices, to learn about neighboring devices and determine whether their interfaces are up or down.

Since CDP does not run on top of any network layer, but rather runs over the data link layer, CDP Protocol Driver (CDP.SYS) is implemented on the Windows platform as an NDIS protocol driver running on top of the existing Ethernet driver.