Cisco Discovery Protocol CDP

Cisco Discovery Protocol (CDP) is a proprietary protocol designed by Cisco to help administrators collect information about both locally attached and remote devices. By using CDP, you can gather hardware and protocol information about neighbor devices, which is useful info for troubleshooting the network.

CDP messages are generated every 60 seconds as multicast messages on each of its active interfaces.
The information shared in a CDP packet about a Cisco device includes the following:

• Name of the device configured with the hostname command
• IOS software version
• Hardware capabilities, such as routing, switching, and/or bridging
• Hardware platform, such as 2600, 2950, or 1900
• The layer-3 address of the device
• The interface the CDP update was generated on

CDP allows devices to share basic configuration information without even configuring any protocol specific information and is enabled by default on all interfaces.
CDP is a Data link Protocol occurring at Layer 2 of the OSI model.
CDP is not routable and can only go over to directly connected devices. 

CDP is enabled, by default, on all Cisco devices. CDP updates are generated as multicasts every 60 seconds with a hold-down period of 180 seconds for a missing neighbor. The no cdp run command globally disables CDP, while the no cdp enable command disables CDP on an interface. Use show cdp neighbors to list out your directly connected Cisco neighboring devices. Adding the detail parameter will display the layer-3 addressing configured on the neighbor.

How could CDP help you?

Kartik has just been hired as a senior network consultant at a large bank in Lucknow, Uttar Pradesh. He is expected to be able to take care of any problem that comes up. No problem at all here—he only has to worry about people possibly not getting the right money transaction if the network goes down. Kartik starts his job happily.

Soon, of course, the network has some problems. He asks one of the junior administrators for a network map so he can troubleshoot the network. This person tells him that the old senior administrator (who just got fired) had them with him and now no one can find them. Cashiers are calling every couple of minutes because they can’t get the necessary information they need to take care of their customers. What should he do?

CDP to the rescue! Thank God this bank has all Cisco routers and switches and that CDP is enabled by default on all Cisco devices. Also, luckily, the dissatisfied administrator who just got fired didn't turn off CDP on any devices before he left. All Kartik has to do now is to use the show cdp neighbor detail command to find all the information he needs about each device to help draw out the bank network .

Cisco Discovery Protocols Configuration commands

Router#show cdp	Displays global CDP information (such as timers)
Router#show cdp neighbors	Displays information about neighbors
Router#show cdp neighbors detail	Displays more detail about the neighbor device
Router#show cdp entry word	Displays information about the device named word
Router#show cdp entry *	Displays information about all devices
Router#show cdp interface	Displays information about interfaces that have CDP running
Router#show cdp interface x	Displays information about specific interface x running CDP
Router#show cdp traffic	Displays traffic information—packets in/out/version
Router(config)#cdp holdtime x	Changes the length of time to keep CDP packets
Router(config)#cdp timer x	Changes how often CDP updates are sent
Router(config)#cdp run	Enables CDP globally (on by default)
Router(config)#no cdp run	Turns off CDP globally
Router(config-if)#cdp enable	Enables CDP on a specific interface
Router(config-if)#cdp enable	Enables CDP on a specific interface
Router(config-if)#no cdp enable	Turns off CDP on a specific interface
Router#clear cdp counters	Resets traffic counters to 0
Router#clear cdp table	Deletes the CDP table
Router#debug cdp adjacency	Monitors CDP neighbor information
Router#debug cdp events	Monitors all CDP events
Router#debug cdp ip	Monitors CDP events specifically for IP
Router#debug cdp packets	Monitors CDP packet-related information