Understanding and Configuring CDP

This chapter describes how to configure Cisco Discovery Protocol (CDP) on the Catalyst 4500 series switch. It also provides guidelines, procedures, and configuration examples.

This chapter includes the following major sections:

•Overview of CDP

•Configuring CDP

Note For complete syntax and usage information for the commands used in this chapter, refer to the
Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2;
Cisco IOS System Management; Configuring Cisco Discovery Protocol (CDP) at this URL:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/ffun_c/fcfprt3/fcf015.htm
and to the Cisco IOS Configuration Fundamentals Command Reference, Release 12.1;
Cisco IOS System Management Commands; and CDP Commands publication at this URL:
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/ffun_r/ffrprt3/frf015.htm

Note For complete syntax and usage information for the switch commands used in this chapter, refer to the Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm.

Overview of CDP

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

CDP runs on all LAN and WAN media that support Subnetwork Access Protocol (SNAP).

Each CDP-configured device sends periodic messages to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain the time-to-live, or holdtime information, which indicates the length of time a receiving device should hold CDP information before discarding it.

Configuring CDP

The following sections describe how to configure CDP:

•Enabling CDP Globally

•Displaying the CDP Global Configuration

•Enabling CDP on an Interface

•Displaying the CDP Interface Configuration

•Monitoring and Maintaining CDP

Enabling CDP Globally

To enable CDP globally, perform this task:

Command
Purpose 
Switch(config)# [no] cdp run

Enables CDP globally.

Use the no keyword to disable CDP globally.


This example shows how to enable CDP globally: 

Switch(config)# cdp run

Displaying the CDP Global Configuration

To display the CDP configuration, perform this task:

Command
Purpose 
Switch# show cdp

Displays global CDP information.


This example shows how to display the CDP configuration: 

Switch# show cdp

Global CDP information:

        Sending CDP packets every 120 seconds

        Sending a holdtime value of 180 seconds

        Sending CDPv2 advertisements is enabled

Switch#

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

Enabling CDP on an Interface

To enable CDP on an interface, perform this task:

Command
Purpose 
Switch(config-if)# [no] cdp enable

Enables CDP on an interface.

Use the no keyword to disable CDP on an interface.


This example shows how to enable CDP on Fast Ethernet interface 5/1: 

Switch(config)# interface fastethernet 5/1

Switch(config-if)# cdp enable

This example shows how to disable CDP on Fast Ethernet interface 5/1: 

Switch(config)# interface fastethernet 5/1

Switch(config-if)# no cdp enable

Displaying the CDP Interface Configuration

To display the CDP configuration for an interface, perform this task:

Command
Purpose 
Switch# show cdp interface [type/number]

Displays information about interfaces where CDP is enabled.


This example shows how to display the CDP configuration of Fast Ethernet interface 5/1: 

Switch# show cdp interface fastethernet 5/1

FastEthernet5/1 is up, line protocol is up

  Encapsulation ARPA

  Sending CDP packets every 120 seconds

  Holdtime is 180 seconds

Switch#

Monitoring and Maintaining CDP

To monitor and maintain CDP on your device, perform one or more of these tasks:

Command
Purpose 
Switch# clear cdp counters

Resets the traffic counters to zero. 

Switch# clear cdp table

Deletes the CDP table of information about neighbors. 

Switch# show cdp

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

Switch# show cdp entry entry_name 
[protocol | version]

Displays information about a specific neighbor. The display can be limited to protocol or version information. 

Switch# show cdp interface 
[type/number]

Displays information about interfaces on which CDP is enabled. 

Switch# show cdp neighbors 
[type/number] [detail]

Displays information about neighboring equipment. The display can be limited to neighbors on a specific interface and expanded to provide more detailed information. 

Switch# show cdp traffic

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

Switch# show debugging

Displays information about the types of debugging that are enabled for your switch.


This example shows how to clear the CDP counter configuration on your switch: 

Switch# clear cdp counters

This example shows how to display information about the neighboring equipment: 

Switch# show cdp neighbors

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

                  S - Switch, H - Host, I - IGMP, r - Repeater


Device ID        Local Intrfce     Holdtme    Capability  Platform  Port ID

JAB023807H1      Fas 5/3            127         T S       WS-C2948  2/46

JAB023807H1      Fas 5/2            127         T S       WS-C2948  2/45

JAB023807H1      Fas 5/1            127         T S       WS-C2948  2/44

JAB023807H1      Gig 1/2            122         T S       WS-C2948  2/50

JAB023807H1      Gig 1/1            122         T S       WS-C2948  2/49

JAB03130104      Fas 5/8            167         T S       WS-C4003  2/47

JAB03130104      Fas 5/9            152         T S       WS-C4003  2/48