Cisco Discovery Protocol (CDP) is a Cisco trademark protocol that is designed to assists the network management of Cisco devices by collecting hardware and software information about the neighboring devices. The network engineers can discover information about neighbor networking devices, determine the type of software version, type of equipment used, interfaces on the device currently active, how they are configured, and other useful information. This information is important when you need to troubleshoot and document network functionality. When you get hired as a Network administrator and are new to the network, the CDP becomes very useful to know about your neighboring devices, their parameters, and other configuration details.

CDP works as a data link layer protocol and uses Subnetwork Access Protocol (SNAP) frame type. SNAP is limited to few media types like Ethernet, Fiber distributed data interfaces(FDDI), Asynchronous Transfer Mode(ATM), Point-to-Point Protocol(PPP), High-Level Data Link Control(HDLC), Token Ring, and Frame Relay. So, CDP also becomes limited to these media types supporting SNAP. CDP frames are distributed as multicasts but are not routed across the device as it is a 2^nd layer protocol.

CDP message frame contains useful information about the network device mentioned below:

IOS version number
Hardware capabilities
Hardware platform
IP address of the device
Interface detail of the device that generates the CDP message
The port ID
Time period the CDP advertisement message is valid. CDP messages are generated in every 60 secs and the holddown time to discover its neighbor is 180 secs.
Device with the hostname
Type of Device

CDP commands used to monitor the router and network is mentioned in the following:

cdp enable, no cdp enable – It is used to enable CDP on the interface in the interface configuration mode. The cdp is enabled by default at the global level on all the supported interfaces. Use “no cdp enable” in the interface configuration mode to disable cdp on the interface.
cdp run, no cdp run – The cdp is always enabled at the global level but due to some reason if it has been disabled, then you can enable it globally by using “cdp run” command in the global configuration mode. To disable cdp at the global level use “no cdp run” command.
cdp timer – It is a global command used to change the default time settings for the CDP timer on the router.
cdp hold time – It is a global command used to change the default time settings for the Holdtime timer on the router.
show cdp neighbors – It displays information about the neighboring network devices.
show cdp neighbors detail – It shows detailed information about the neighboring network devices.
show cdp entry – It works the same as the “show cdp neighbors” command and displays information about neighboring devices.
show cdp entry [device name] – It shows information about the device as specified.
show cdp interface – It shows cdp status on router interfaces.
show cdp traffic – It shows cdp traffic information.
clear cdp counter – It resets the cdp traffic counter data to zero.
clear cdp table– It clears the cdp tables information.