CDP (Cisco Discovery Protocol) Resource
The Cisco Discovery Protocol (CDP) is a media-independent device discovery protocol that runs on all Cisco manufactured equipment, including routers, bridges, access servers, and switches.
In addition, network management applications can retrieve the device type and SNMP-agent address of neighboring Cisco devices. This enables applications to send SNMP queries to neighboring devices. CDP allows network management applications to discover Cisco devices that are neighbors of already known devices, in particular, neighbors running lower-layer, transparent protocols.
CDP runs on all media that support Subnetwork Access Protocol (SNAP), including LAN and Frame Relay. CDP runs over the data link layer only.
Cisco devices never forward CDP packets. When new CDP information is received, Cisco devices discard old information.