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This chapter describes how to configure Cisco Discovery Protocol (CDP) on the switch. Unless otherwise noted, the term switch refers to a standalone switch and to a 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 12.2.
This chapter consists of these sections:
•Monitoring and Maintaining 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 SNMP management applications to display a graphical view of the network.
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.
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.
These sections contain this configuration information:
•Configuring the CDP Characteristics
•Disabling and Enabling CDP on an Interface
Table 27-1 shows the default CDP configuration.
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.
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.
CDP is enabled by default.
Beginning in privileged EXEC mode, follow these steps to disable the CDP device discovery capability:
|
|
|
---|---|---|
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:
|
|
|
---|---|---|
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
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:
Beginning in privileged EXEC mode, follow these steps to enable CDP on a port when it has been disabled:
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
Note Voice VLAN is not counted against port security when CDP is disabled on the switch interface.
To monitor and maintain CDP on your device, use one or more of the privileged EXEC commands in Table 27-2.
Table 27-2 Commands for Displaying CDP Information