Software Configuration Guide, Cisco IOS Release 15.2(2)E (Industrial Ethernet 3000 Switch)

Configuring CDP

---

Note This chapter describes how to configure Cisco Discovery Protocol (CDP) on the IE 3000 switch. 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 12.2.

This chapter consists of these sections:

• Understanding CDP
• Configuring CDP
• Monitoring and Maintaining CDP

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, go to:
http://www.cisco.com/en/US/docs/ios/netmgmt/configuration/guide/nm_cdp_discover.html.

The switch supports CDP Version 2.

Configuring CDP

These sections contain this configuration information:

• Default CDP Configuration
• Configuring the CDP Characteristics
• Disabling and Enabling CDP
• Disabling and Enabling CDP on an Interface

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”.

---

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 gigabitethernet1/1

Switch(config-if)# cdp enable

Switch(config-if)# end

---

Note Voice VLAN is not counted against port security when CDP is disabled on the switch interface.

---

Monitoring and Maintaining CDP

To monitor and maintain CDP on your device, perform one or more of these tasks, beginning in privileged EXEC mode.