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Cisco Discovery Protocol (CDP) advertises
device information to its direct neighbors, including MAC and IP
addresses and outgoing interfaces. The output from the
show cdp neighbors
command displays information about directly connected neighbors.
 This
information is useful for debugging connectivity issues. If a cabling
problem is suspected, enable the interfaces with the
no shutdown
command and then execute the
show cdp neighbors detail
command before any other configuration. The command displays specific
device detail such as the active interfaces, the port ID, and the
device. The version of Cisco IOS that is running on the remote devices
is also shown.
If the physical layer is properly
functioning, then all other directly connected Cisco devices should be
displayed. If no known device appears, a Layer 1 problem likely
exists.
One area of concern with CDP is
security. The amount of information CDP provides is so extensive that
it can be a potential security hole.
 For
security reasons CDP should be configured only on links between Cisco
devices and disabled on user ports or links that are not locally
managed.
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Lab Activity
e-Lab Activity: Cisco Discovery Protocol
In this lab, the students will use CDP to obtain information
about neighboring Cisco devices.
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Web Links