Cisco Discovery Protocol (CDP) is a Cisco proprietary (layer 2) protocol that provides information about directly connected Cisco routers and switches (if enabled on the switch). CDP messages are not forwarded and, of course, not routed (layer 2). The protocol operates transparently and is enabled by default on all Cisco routers. It can be useful if you are trying to troubleshoot a connectivity problem. If the Data Link layer is operating, you will see CDP information from all directly connected routers (and switches, if enabled) that also have layer 2 functionality. Here is the kind of information provided:

ROUTER#show cdp neighbors detail
-------------------------
Device ID: RTR3
Entry address(es):
IP address: 10.10.10.2
Novell address: a1.0090.ab80.5e0f
(Other configured network protocols would appear here)
Platform: cisco 2500, Capabilities: Router
Interface: Serial0.103, Port ID (outgoing port): Serial0.1
Holdtime : 151 sec
Version :
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JOS56I-L), Version 12.0(7),
RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Thu 14-Oct-99 01:54 by phanguye

CDP Timers and Holdtime Information

The show cdp command gives you information about two CDP global parameters that can be configured on Cisco devices:

CDP timer is how often CDP packets are transmitted to all active interfaces.
CDP holdtime is the amount of time that the device will hold packets received from neighbor devices.

Both Cisco routers and Cisco switches use the same parameters.

The output on a router looks like this:
Router#sh cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds

Use the global commands cdp holdtime and cdp timer to configure the CDP holdtime and timer on a router:
ROUTER#config t
ROUTER(config)#cdp timer 90
ROUTER(config)#cdp holdtime 240
ROUTER(config)#^Z

You can turn off CDP completely with the no cdp run command from the global configuration mode of a router. To turn CDP off or on for an interface, use the no cdp enable and cdp enable commands.

The show cdp neighbor command delivers information about directly connected devices.

show cdp neighbor detail command can be run on both routers and switches, and it displays detailed information about each device connected to the device you're running the command on.

The show cdp entry * command displays the same information as the show cdp neighbor details command.