Cisco CCNA Exam Tutorial: Cisco Discovery Protocol (CDP)
The Cisco Discovery Protocol (CDP) sure looks simple enough, but there are quite a few details
to know for success on the 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
R2#show cdp neighbor
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r – Repeater
Device ID Local Intrfce Holdtme Capability Platform Port ID
R1 BRI0 167 R 2521 Dialer1
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.
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
CDP is running by default.
R2(config)#cdp timer 45