Cisco Discovery Protocol

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The CDP ( Cisco Discovery Protocol ) is a protocol that enables a network component to automatically create a list of directly connected devices that also support CDP. It was developed by the Cisco company and is mainly used on Cisco devices. It acts on the LLC plane of the link layer (layer 2) in the OSI model . If the underlying network access method supports the Subnetwork Access Protocol , it is media and protocol independent and can therefore be used with Ethernet , FDDI , ATM and other methods.

Every device that is configured for CDP sends periodic messages (advertisements) to an Ethernet multicast address (01-00-0C-CC-CC-CC) and receives messages from neighboring devices. Each message contains information such as the device name, the operating system version, interface identifier, management IP addresses and the holdtime of the packet. If the device information is not periodically updated, the old information is discarded after the hold time has expired. In the standard setting, Cisco routers send a CDP packet with a holdtime of 180 seconds every 60 seconds on almost all interfaces that support CDP.

CDP has no security mechanisms. CDP messages can easily be forged and eavesdropped.

The current version of CDP is CDPv2. In contrast to the first version, this enables faster troubleshooting through more information.

A comparable, manufacturer-independent protocol is LLDP .

Commands for configuring the CDP via Cisco IOS (excerpt)

  • Display the status of the CDP:
show cdp
  • global activation of the CDP process:
cdp run
  • Activating CDP for a special interface:
cdp enable
  • Make settings for the sending interval of CDP updates:
cdp timer
  • Make settings for the time a CDP packet is held:
cdp holdtime
  • Delete table with CDP information:
clear cdp table
  • To view information about CDP traffic:
show cdp traffic
  • Show information about neighboring CDP devices:
show cdp neighbors
  • To view information about a specific CDP device:
show cdp entry
  • Display information about the interfaces on which CDP is activated:
show cdp interface