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CCNA Tutorial: Cisco Discovery Protocol (CDP)
By Chris Bryant, CCIE #12933
For a protocol that runs by default and isn't often changed from those defaults in real-world networks, there are quite a few details you better know about CDP before taking your CCNA exam. In your CCNP studies, you'll be introduced to additional uses for CDP, but for now it's enough to know that CDP is designed to give you information regarding directly connected Cisco routers and switches.
CDP runs by default between all directly connected Cisco devices. CDP is also a Cisco-proprietary protocol - if the directly connected device is not a Cisco device, you won't see the information you wanted.
The basic CDP command to display information about the directly connected neighbor is show cdp neighbor.
This command is particularly helpful when troubleshooting Cisco switches. There’s no need to trace wiring in a rack of Cisco devices to see what routers are connected to a Cisco switch when show cdp neighbor can be used. In the above output, you can see the remote device's hostname, what interface on the remote device is connected to the local device, the capability of the remote device, the remote device’s hardware platform, and the local interface that is connected to the remote device.
CDP can be disabled at both the global and interface level. To disable CDP at the interface level, run no cdp enable on the interface, and cdp enable to turn it back on.
cdp timer defines how often CDP packets are transmitted, and cdp holdtime defines how long a device will hold a received packet.
To turn CDP off for the entire router, run no cdp run. To view the current global status of CDP, run show cdp.
Finally, there's one more CDP command you really should know, especially for real-world networks. show cdp neighbor gives you a lot of information, but there's one vital piece of info missing - the neighbor's IP address. You'll need to run show cdp neighbor detail to get that. (The version of that command you see below uses a modifier to go to a line including "R2"; you don't have to know that variation for the CCNA exam, but that modifier does come in handy.) And to navigate a poorly documented network, believe me, you're going to need that command!
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