Configuring CDP

This chapter describes how to configure Cisco Discovery Protocol (CDP) on the Catalyst 3750-X or 3560-X switch. Unless otherwise noted, the term switch refers to a Catalyst 3750-X or 3560-X standalone switch and to a Catalyst 3750-X switch stack.

Note For complete syntax and usage information for the commands used in this chapter, see the command reference for this release and the “System Management Commands” section in the Cisco IOS Configuration Fundamentals Command Reference, Release 12.4.

Understanding CDP

CDP is a device discovery protocol that runs over Layer 2 (the data link layer) on all Cisco-manufactured devices (routers, bridges, access servers, 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.

For a switch and connected endpoint devices running Cisco Medianet

  • CDP identifies connected endpoints that communicate directly with the switch.
  • To prevent duplicate reports of neighboring devices, only one wired switch reports the location information.
  • The wired switch and the endpoints both send and receive location information.

For information:
http://www.cisco.com/en/US/docs/ios/netmgmt/configuration/guide/nm_cdp_discover.html.

The switch supports CDP Version 2.

CDP and Switch 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.

Configuring CDP

Default CDP Configuration

 

Table 30-1 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

Configuring the CDP Characteristics

You can configure the frequency of CDP updates, the amount of time to hold the information before discarding it, and whether or not to send Version-2 advertisements.

Beginning in privileged EXEC mode, follow these steps to configure the CDP timer, holdtime, and advertisement type.

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

 

Command
Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

cdp timer seconds

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

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

Step 3

cdp holdtime seconds

(Optional) Specify 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 4

cdp advertise-v2

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

This is the default state.

Step 5

end

Return to privileged EXEC mode.

Step 6

show cdp

Verify your settings.

Step 7

copy running-config startup-config

(Optional) Save your entries in the configuration file.

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

This 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
 

For additional CDP show commands, see the “Monitoring and Maintaining CDP” section.

Disabling and 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. For more information, see Chapter 6, “Clustering Switches” and see Getting Started with Cisco Network Assistant, available on Cisco.com.

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

 

Command
Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

no cdp run

Disable CDP.

Step 3

end

Return to privileged EXEC mode.

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

 

Command
Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

cdp run

Enable CDP after disabling it.

Step 3

end

Return to privileged EXEC mode.

This example shows how to enable CDP if it has been disabled.

Switch# configure terminal
Switch(config)# cdp run
Switch(config)# end

Disabling and Enabling CDP on an Interface

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

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

 

Command
Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

interface interface-id

Specify the interface on which you are disabling CDP, and enter interface configuration mode.

Step 3

no cdp enable

Disable CDP on the interface.

Step 4

end

Return to privileged EXEC mode.

Step 5

copy running-config startup-config

(Optional) Save your entries in the configuration file.

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

 

Command
Purpose

Step 1

configure terminal

Enter global configuration mode.

Step 2

interface interface-id

Specify the interface on which you are enabling CDP, and enter interface configuration mode.

Step 3

cdp enable

Enable CDP on the interface after disabling it.

Step 4

end

Return to privileged EXEC mode.

Step 5

copy running-config startup-config

(Optional) Save your entries in the configuration file.

This example shows how to enable CDP on a port when it has been disabled.

Switch# configure terminal
Switch(config)# interface gigabitethernet1/0/1
Switch(config-if)# cdp enable
Switch(config-if)# end

Monitoring and Maintaining CDP

Table 30-2 Commands for Displaying CDP Information

 

Command
Description

clear cdp counters

Reset the traffic counters to zero.

clear cdp table

Delete the CDP table of information about neighbors.

show cdp

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

show cdp entry entry-name [ protocol | version ]

Display 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 ]

Display 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 ]

Display 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

Display CDP counters, including the number of packets sent and received and checksum errors.