Cisco Discovery Protocol (CDP) is a Layer 2 discovery protocol. Cisco devices send out CDP packets every thirty seconds (they are sent as multicast packets). Cisco devices, including switches, do not forward these packets, so only devices on the segment the packets were sent on see them. Cisco devices record the information in a CDP table that can be queried by the cdpCacheTable in the CISCO-CDP-MIB or shown by the show cdp neighbors [detail] CLI command. This table includes the following information:
This information can be used to determine the Layer 2 topology of your network. Knowledge of the Layer 2 topology of your network can be used for many purposes, including Layer 2 fault correlation and determining what information is redundant for performance collection and analysis.
Note, however, that non-Cisco devices do not support CDP and will not be discovered by CDP. If a link between two Cisco devices transits one or more non-Cisco devices, the type of devices transited will determine the result. Non-Cisco routers will block the CDP multicasts. Non-Cisco switches or hubs will transmit or flood the CDP packets and, thus, will be transparent to CDP.
All Cisco IOS devices have supported this protocol since release 10.3. Cisco Catalyst devices have supported CDP since the initial software release.