What is CDP?

CDP is the Cisco Discovery Protocol. It is a Cisco Proprietary Protocol that discovers and shows information about directly connected Cisco devices. CDP works on layer two (Data Link layer) of the OSI Reference Model.
Cisco Discovery Protocol (CDP) works only with the Cisco manageable devices such as Cisco Catalyst Switches and Cisco Routers and Access Server. It is a useful utility of Cisco IOS that runs on the routers with the Cisco IOS version 10.3 or later and Cisco switches and hubs. CDP is basically network monitor software that is used for managing your network environment. It gives you a summary of some important information about neighbour and remote Cisco devices that are implemented in your network. CDP discovers neighbour devices regardless of which protocol suite they are running. Physical media must support the SNAP (Subnetwork Access Protocol) encapsulation. Cisco also offers the PC generating Cisco Discovery Protocol to provide the summary information about PC that are connected with Cisco devices and the Cisco VoIP Cisco Discovery Protocol for providing the summary information of Cisco Voice integrated routers.

The summary information includes:

  • Device Identifiers
  • Address List
  • Port Identifier
  • Capabilities List
  • Platform

For better understanding the Cisco Discovery Protocol (CDP), we should have to discuss the protocol in layering element.

The layering elements of CDP are as follows:

  • Physical layer: Media supporting SNAP such as LANs, Frame Relay ATM and others
  • Data Link Layer: Cisco Proprietary Data Link protocol that discovers and shows information about directly and remotely connected devices
  • Network Layer: Upper layer entry addresses of any protocol suite such as TCP/IP, Novell IPX/SPX, Apple Talk and others

Configuring CDP on a Cisco Router

CDP Cisco runs on the routers with Cisco IOS 10.3 or later and Cisco Catalyst switches and hubs. Cisco cdp has two versions for sending CDP advertisements on Cisco neighbour devices such as CDP version 1 and CDP version 2.
CDP is by default enabled in Cisco routers and switches with Cisco IOS 10.3 or later. You can verify the status of CDP by following privileged-exec mode command:

  • Router # show cdp

The output of the command shows that CDP is running on the router. If you are not interested to run CDP on your router for some reasons then you have to execute this set of command:

  • Router # configure terminal
  • Router (config) # no cdp run
  • Router # show cdp

Execute the following set of command to run CDP on the router on which CDP is currently not running:

  • Router # configure terminal
  • Router (config) # cdp run
  • Router # show cdp

By default, the CDP advertisements are sending after every 60 seconds by an update CDP packet with a hold time value of 180 seconds. You can change the default configuration by these set of commands:

  • Router # configure terminal
  • Router (config) # cdp timer 30
  • Router (config) # cdp hold time 90
  • Router # show cdp

You can configure the cdp timer value from 5 to 254 seconds and cdp hold time value from 10 to 255 seconds. It is recommended that the hold time value should be three times greater than the value of cdp timer.

To disable the Cisco Discovery Protocol on a specific interface, you have to perform the following interface configuration command:

  • Router (config) # interface serial 0
  • Router (config-if) # no cdp enable
  • Router # show cdp interface

For enabling the Cisco Discovery Protocol on a specific interface, you have to perform the following interface configuration command:

  • Ruter (config) # interface serial 0
  • Router (config-if) # cdp enable
  • Router # show cdp interface

There are some other show commands are available in the privileged mode of the Cisco IOS for verifying the CDP configuration.

Followings are the commands and their short description used for verifying the CDP:

  • R1 # show cdp entry R2: displays CDP information for specific neighbour entry
  • R1 # show cdp entry : displays CDP information for all the neighbour entries
  • R1 # show cdp interface: displays the CDP status and configuration of interfaces
  • R1 # show cdp traffic: displays CDP traffic statistics
  • R1 # show cdp neighbours: displays CDP neighbour entries
  • R1 # show cdp neighbours detail: displays the detailed CDP neighbours entries

CDP is also by default enabled in Cisco Catalyst switches such as Cisco Catalyst 1900 series switch and Cisco Catalyst 2950 series switch. Cisco Catalyst 1900 series switch also provides its MAC address in the summary information through the output of show cdp neighbours command.

Cisco VoIP Cisco Discovery Protocol

Voice and data networks have traditionally been two separated networks. Over the few years, many organizations have begun merging the voice and data network. This trend is gradually accelerating and it will continue to accelerate for coming years. This technology is called Voice over IP (VOIP).

Cisco also provides the hardware and software solution of Voice over IP networks. Cisco offers voice modules in different router models such as Cisco 2610 series. If two or more Cisco routers having voice modules are connected via voice port such as FXO port or FXS port, then the Cisco VoIP Cisco Discovery Protocol displays the summary information of neighbour Cisco devices.

The summary information includes:

  • Device Identifiers such as the name of the Cisco Voice router
  • Address List such as the IP address of the Voice interface of the Cisco router
  • Port Identifier such as the incoming and outgoing Voice interfaces that are using to connect two Cisco routers
  • Capabilities List such as the capabilities of the Cisco Voice device.
  • Platform such as the series and model no of Cisco Voice router.