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.

The following applies to 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.

The switch supports CDP Version 2.

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.

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
Note

Steps 3 through 5 are all optional and can be performed in any order.

Follow these steps to configure the CDP characteristics.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

cdp 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 4

cdp 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 5

cdp advertise-v2

Example:


Switch(config)# cdp advertise-v2

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

This is the default state.

Step 6

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 7

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 8

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

What to do next

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

Disabling CDP

CDP is enabled by default.

Note
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.

Follow these steps to disable the CDP device discovery capability.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

no cdp run

Example:

Switch(config)# no cdp run

Disables CDP.

Step 4

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 5

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 6

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

What to do next

You must reenable CDP to use it.

Enabling CDP

CDP is enabled by default.

Note
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.

Follow these steps to enable CDP when it has been disabled.

Before you begin

CDP must be disabled, or it cannot be enabled.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

cdp run

Example:

Switch(config)# cdp run

Enables CDP if it has been disabled.

Step 4

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 5

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 6

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

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.

Disabling CDP on an Interface

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

Note
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.

Follow these steps to disable CDP on a port.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

interface interface-id

Example:

Switch(config)# interface gigabitethernet1/0/1

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

Step 4

no cdp enable

Example:

Switch(config-if)# no cdp enable

Disables CDP on the interface specified in Step 3.

Step 5

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 6

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 7

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

Enabling CDP on an Interface

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

Note
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.

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.

Procedure

  Command or Action Purpose
Step 1

enable

Example:


Switch> enable

Enables privileged EXEC mode. Enter your password if prompted.

Step 2

configure terminal

Example:


Switch# configure terminal

Enters the global configuration mode.

Step 3

interface interface-id

Example:

Switch(config)# interface gigabitethernet1/0/1

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

Step 4

cdp enable

Example:

Switch(config-if)# cdp enable

Enables CDP on a disabled interface.

Step 5

end

Example:


Switch(config)# end

Returns to privileged EXEC mode.

Step 6

show running-config

Example:


Switch# show running-config

Verifies your entries.

Step 7

copy running-config startup-config

Example:


Switch# copy running-config startup-config

(Optional) Saves your entries in the configuration file.

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.

Additional References

Related Documents

Related Topic Document Title

System Management Commands

Command Reference, Cisco IOS Release 15.2(2)E

Error Message Decoder

Description Link

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https://www.cisco.com/cgi-bin/Support/Errordecoder/index.cgi

Standards and RFCs

Standard/RFC Title

None

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MIBs

MIB MIBs Link

All supported MIBs for this release.

To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

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Feature History and Information for Cisco Discovery Protocol

Release

Modification

This feature was introduced.