Configuring the Cisco Discovery Protocol

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Information About CDP

CDP Overview

CDP is a device discovery protocol that runs over Layer 2 (the data-link layer) on all Cisco-manufactured devices (routers, bridges, access servers, controllers, and switches) and allows network management applications to discover Cisco devices that are neighbors of already known devices. With CDP, network management applications can learn the device type and the Simple Network Management Protocol (SNMP) agent address of neighboring devices running lower-layer, transparent protocols. This feature enables applications to send SNMP queries to neighboring devices.

CDP runs on all media that support Subnetwork Access Protocol (SNAP). Because CDP runs over the data-link layer only, two systems that support different network-layer protocols can learn about each other.

Each CDP-configured device sends periodic messages to a multicast address, advertising at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime information, which is the length of time a receiving device holds CDP information before discarding it. Each device also listens to the messages sent by other devices to learn about neighboring devices.

On the switch, CDP enables Network Assistant to display a graphical view of the network. The switch uses CDP to find cluster candidates and maintain information about cluster members and other devices up to three cluster-enabled devices away from the command switch by default.

CDP and Stacks

A switch stack appears as a single switch in the network. Therefore, CDP discovers the switch stack, not the individual stack members. The switch stack sends CDP messages to neighboring network devices when there are changes to the switch stack membership, such as stack members being added or removed.

Default CDP Configuration

This table shows the default CDP configuration.

Feature Default Setting
CDP global state Enabled
CDP interface state Enabled
CDP timer (packet update frequency) 60 seconds
CDP holdtime (before discarding) 180 seconds
CDP Version-2 advertisements Enabled

How to Configure CDP

Configuring CDP Characteristics

You can configure these CDP characteristics:

  • Frequency of CDP updates

  • Amount of time to hold the information before discarding it

  • Whether or not to send Version-2 advertisements


Note

Steps 2 through 4 are all optional and can be performed in any order.

Beginning in privileged EXEC mode, follow these steps to configure these characteristics.

SUMMARY STEPS

    1.    configure terminal

    2.    cdp timer seconds

    3.    cdp holdtime seconds

    4.    cdp advertise-v2

    5.    end


DETAILED STEPS
     Command or ActionPurpose
    Step 1configure terminal


    Example:
    
Switch# configure terminal
     

    Enters the global configuration mode.

     
    Step 2cdp timer seconds


    Example: 
    
Switch(config)# cdp timer 20
     

    (Optional) Sets the transmission frequency of CDP updates in seconds.

    The range is 5 to 254; the default is 60 seconds.

     
    Step 3cdp holdtime seconds


    Example:
    Switch(config)# cdp holdtime 60
     

    (Optional) Specifies the amount of time a receiving device should hold the information sent by your device before discarding it.

    The range is 10 to 255 seconds; the default is 180 seconds.

     
    Step 4cdp advertise-v2


    Example:
    Switch(config)# cdp advertise-v2
     

    (Optional) Configures CDP to send Version-2 advertisements.

    This is the default state.

     
    Step 5end


    Example:
    
Switch(config)# end
     

    Returns to privileged EXEC mode.

     

    Example

    The following example shows how to configure CDP characteristics:

    Switch# configure terminal
Switch(config)# cdp timer 50
Switch(config)# cdp holdtime 120
Switch(config)# cdp advertise-v2
Switch(config)# end
    What to Do Next

    Use the no form of the CDP commands to return to the default settings.

    Related References
    Monitoring and Maintaining CDP

    Disabling CDP

    CDP is enabled by default.


    Note
    Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity.

    Beginning in privileged EXEC mode, follow these steps to disable the CDP device discovery capability.

    SUMMARY STEPS

      1.    configure terminal

      2.    no cdp run

      3.    end


    DETAILED STEPS
       Command or ActionPurpose
      Step 1configure terminal


      Example:
      
Switch# configure terminal
       

      Enters the global configuration mode.

       
      Step 2no cdp run


      Example:
      Switch(config)# no cdp run
       

      Disables CDP.

       
      Step 3end


      Example:
      
Switch(config)# end
       

      Returns to privileged EXEC mode.

       
      What to Do Next

      You must reenable CDP to use it.

      Related Tasks
      Enabling CDP

      Enabling CDP

      CDP is enabled by default.


      Note
      Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity.

      Beginning in privileged EXEC mode, follow these steps to enable CDP when it has been disabled.

      Before You Begin

      CDP must be disabled, or it cannot be enabled.

      SUMMARY STEPS

        1.    configure terminal

        2.    cdp run

        3.    end


      DETAILED STEPS
         Command or ActionPurpose
        Step 1configure terminal


        Example:
        
Switch# configure terminal
         

        Enters the global configuration mode.

         
        Step 2cdp run


        Example:
        Switch(config)# cdp run
         

        Enables CDP if it has been disabled.

         
        Step 3end


        Example:
        
Switch(config)# end
         

        Returns to privileged EXEC mode.

         

        Example

        The following example shows how to enable CDP if it has been disabled:

        Switch# configure terminal
Switch(config)# cdp run
Switch(config)# end
        What to Do Next

        Use the show run all command to show that CDP has been enabled. If you enter only show run, the enabling of CDP may not be displayed.

        Related Tasks
        Disabling CDP

        Disabling CDP on an Interface

        CDP is enabled by default on all supported interfaces to send and to receive CDP information.


        Note
        Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity.

        Beginning in privileged EXEC mode, follow these steps to disable CDP on a port.

        SUMMARY STEPS

          1.    configure terminal

          2.    interface interface-id

          3.    no cdp enable

          4.    end


        DETAILED STEPS
           Command or ActionPurpose
          Step 1configure terminal


          Example:
          
Switch# configure terminal
           

          Enters the global configuration mode.

           
          Step 2interface interface-id


          Example:
          Switch(config)# interface gigabitethernet1/0/1
           

          Specifies the interface on which you are disabling CDP, and enters interface configuration mode.

           
          Step 3no cdp enable


          Example:
          Switch(config-if)# no cdp enable
           

          Disables CDP on the interface specified in Step 2.

           
          Step 4end


          Example:
          Switch(config-if)# end
           

          Returns to privileged EXEC mode.

           
          Related Tasks
          Enabling CDP on an Interface

          Enabling CDP on an Interface

          CDP is enabled by default on all supported interfaces to send and to receive CDP information.


          Note
          Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity.

          Beginning in privileged EXEC mode, follow these steps to enable CDP on a port on which it has been disabled.

          Before You Begin

          CDP must be disabled on the port that you are trying to CDP enable on, or it cannot be enabled.

          SUMMARY STEPS

            1.    configure terminal

            2.    interface interface-id

            3.    cdp enable

            4.    end


          DETAILED STEPS
             Command or ActionPurpose
            Step 1configure terminal


            Example:
            
Switch# configure terminal
             

            Enters the global configuration mode.

             
            Step 2interface interface-id


            Example:
            Switch(config)# interface gigabitethernet1/0/1
             

            Specifies the interface on which you are enabling CDP, and enters interface configuration mode.

             
            Step 3cdp enable


            Example:
            Switch(config-if)# cdp enable
             

            Enables CDP on a disabled interface.

             
            Step 4end


            Example:
            Switch(config-if)# end
             

            Returns to privileged EXEC mode.

             

            Example

            The following example shows how to enable CDP on a disabled port:

            Switch# configure terminal
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# cdp enable
Switch(config-if)# end
            Related Tasks
            Disabling CDP on an Interface

            Monitoring and Maintaining CDP

            Table 1 Commands for Displaying CDP Information
            Command Description

            clear cdp counters

            Resets the traffic counters to zero.

            clear cdp table

            Deletes the CDP table of information about neighbors.

            show cdp

            Displays global information, such as frequency of transmissions and the holdtime for packets being sent.

            show cdp entry entry-name [version] [protocol]

            Displays information about a specific neighbor.

            You can enter an asterisk (*) to display all CDP neighbors, or you can enter the name of the neighbor about which you want information.

            You can also limit the display to information about the protocols enabled on the specified neighbor or information about the version of software running on the device.

            show cdp interface [interface-id]

            Displays information about interfaces where CDP is enabled.

            You can limit the display to the interface about which you want information.

            show cdp neighbors [interface-id] [detail]

            Displays information about neighbors, including device type, interface type and number, holdtime settings, capabilities, platform, and port ID.

            You can limit the display to neighbors of a specific interface or expand the display to provide more detailed information.

            show cdp traffic

            Displays CDP counters, including the number of packets sent and received and checksum errors.
            Related Tasks
            Configuring CDP Characteristics

            Additional References

            Related Documents

            Related Topic Document Title

            System Management Commands

            Configuration Fundamentals Command Reference, Cisco IOS XE Release 3S (Catalyst 3850 Switches)

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            Feature History and Information for Cisco Discovery Protocol

            Release

            Modification

            Cisco IOS XE 3.3SE

            This feature was introduced.