This is a very useful command for troubleshooting or verifying connectivity to directly connected devices. The physical media must support SNAP (subnetwork access protocol). When you issue the cdp commands, the output display gives a summary of the protocol and address information for the Cisco devices, as well as the devices' hardware and software information.
CDP is a Cisco proprietary layer 2 only protocol for Ethernet and Serial.
Question: Note to self, what does this really mean? Devices connected on the fast ethernet and serial ports!
Answer: This means that it does not need a configured IP address to function (layer 3).
usage: show cdp
This will load information about timers, perhaps not quite what you want.
List of Device Identifiers (Variables)
usage: show cdp neighbor
Device ID: the hostname of the directly attached device
Local interface: the port identifier where the device is directly connected to
Hold time: the amount of time the device will hold the CDP information before discarding
Capability: identified as router, switch, hub, repeater
Hardware platform: Cisco series
Port ID: port on the remote device that this device is attached to
Commands gives additional and more complete information about the neighbor; both the following commands display the same output
usage: show cdp neighors detail
usage: show cdp entry hostname
Issue the commands in Global Configuration mode
usage: show cdp ?
entry - gives info about specific device
interface - displays interfaces enabled with CDP and other parameters such as encapsulation, status and configuration
neighbors - CDP neighbor entries
traffic - CDP statistics
Configuring CDP with Security In Mind
usage: no cdp run - issue in global conf mode, to turn off CDP globally; prevents other CDP capable devices from accessing info on this device
usage: no cdp enable - disables cdp on a particular interface; recommended to turn off cdp on the interface facing the WAN side.
usage: cdp enable - enables cdp on the interface!
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