Configuring the Cisco Discovery Protocol

Cisco Discovery Protocol is a Layer 2, media-independent, and network-independent protocol that runs on Cisco devices and enables networking applications to learn about directly connected devices nearby. This protocol facilitates the management of Cisco devices by discovering these devices, determining how they are configured, and allowing systems using different network-layer protocols to learn about each other.

This module describes Cisco Discovery Protocol Version 2 and how it functions with SNMP.

Information About CDP

Cisco Discovery Protocol Overview

Cisco Discovery Protocol 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 Cisco Discovery Protocol, network management applications can learn the device type and the SNMP agent address of neighboring devices running lower-layer, transparent protocols. This feature enables applications to send SNMP queries to neighboring devices.

Cisco Discovery Protocol runs on all media that support Subnetwork Access Protocol (SNAP). Because Cisco Discovery Protocol runs over the data-link layer only, two systems that support different network-layer protocols can learn about each other.

Each Cisco Discovery Protocol-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 Cisco Discovery Protocol information before discarding it. Each device also listens to the messages sent by other devices to learn about neighboring devices.

On the device, Cisco Discovery Protocol enables Network Assistant to display a graphical view of the network. The device uses Cisco Discovery Protocol to find cluster candidates and maintain information about cluster members and other devices up to three cluster-enabled devices away from the command device by default.

  • Cisco Discovery Protocol identifies connected endpoints that communicate directly with the device.

  • To prevent duplicate reports of neighboring devices, only one wired device reports the location information.

  • The wired device and the endpoints both send and receive location information.

Default Cisco Discovery Protocol Configuration

This table shows the default Cisco Discovery Protocol configuration.

Feature Default Setting
Cisco Discovery Protocol global state Enabled
Cisco Discovery Protocol interface state Enabled
Cisco Discovery Protocol timer (packet update frequency) 60 seconds
Cisco Discovery Protocol holdtime (before discarding) 180 seconds
Cisco Discovery Protocol Version-2 advertisements Enabled

How to Configure CDP

Configuring Cisco Discovery Protocol Characteristics

You can configure these Cisco Discovery Protocol characteristics:

  • Frequency of Cisco Discovery Protocol updates

  • Amount of time to hold the information before discarding it

  • Whether or not to send Version 2 advertisements


Note

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

Follow these steps to configure the Cisco Discovery Protocol characteristics.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. cdp timer seconds
  4. cdp holdtime seconds
  5. cdp advertise-v2
  6. end
  7. show running-config
  8. copy running-config startup-config

DETAILED STEPS

  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 global configuration mode.

Step 3

cdp timer seconds

Example:


Switch(config)# cdp timer 20

(Optional) Sets the transmission frequency of Cisco Discovery Protocol 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 Cisco Discovery Protocol 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 Cisco Discovery Protocol commands to return to the default settings.

Disabling Cisco Discovery Protocol

Cisco Discovery Protocol is enabled by default.


Note
Device clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange Cisco Discovery Protocol messages. Disabling Cisco Discovery Protocol can interrupt cluster discovery and device connectivity.

Follow these steps to disable the Cisco Discovery Protocol device discovery capability.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. no cdp run
  4. end
  5. show running-config
  6. copy running-config startup-config

DETAILED STEPS

  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 global configuration mode.

Step 3

no cdp run

Example:

Switch(config)# no cdp run

Disables Cisco Discovery Protocol.

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 Cisco Discovery Protocol to use it.

Enabling Cisco Discovery Protocol

Cisco Discovery Protocol is enabled by default.


Note
Device clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange Cisco Discovery Protocol messages. Disabling Cisco Discovery Protocol can interrupt cluster discovery and device connectivity.

Follow these steps to enable Cisco Discovery Protocol when it has been disabled.

Before you begin

Cisco Discovery Protocol must be disabled, or it cannot be enabled.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. cdp run
  4. end
  5. show running-config
  6. copy running-config startup-config

DETAILED STEPS

  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 global configuration mode.

Step 3

cdp run

Example:

Switch(config)# cdp run

Enables Cisco Discovery Protocol 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 Cisco Discovery Protocol has been enabled. If you enter only show run , the enabling of Cisco Discovery Protocol may not be displayed.

Disabling Cisco Discovery Protocol on an Interface

Cisco Discovery Protocol is enabled by default on all supported interfaces to send and to receive Cisco Discovery Protocol information.


Note
Device clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange Cisco Discovery Protocol messages. Disabling Cisco Discovery Protocol can interrupt cluster discovery and device connectivity.

Note
Cisco Discovery Protocol bypass is not supported and may cause a port go into err-disabled state.

Follow these steps to disable Cisco Discovery Protocol on a port.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface interface-id
  4. no cdp enable
  5. end
  6. show running-config
  7. copy running-config startup-config

DETAILED STEPS

  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 global configuration mode.

Step 3

interface interface-id

Example:

Switch(config)# interface gigabitethernet 1/0/1

Specifies the interface on which you are disabling Cisco Discovery Protocol, and enters interface configuration mode.

Step 4

no cdp enable

Example:

Switch(config-if)# no cdp enable

Disables Cisco Discovery Protocol 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 Cisco Discovery Protocol on an Interface

Cisco Discovery Protocol is enabled by default on all supported interfaces to send and to receive Cisco Discovery Protocol information.


Note
Device clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange Cisco Discovery Protocol messages. Disabling Cisco Discovery Protocol can interrupt cluster discovery and device connectivity.

Note
Cisco Discovery Protocol bypass is not supported and may cause a port go into err-disabled state.

Follow these steps to enable Cisco Discovery Protocol on a port on which it has been disabled.

Before you begin

Cisco Discovery Protocol must be disabled on the port that you are trying to Cisco Discovery Protocol enable on, or it cannot be enabled.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface interface-id
  4. cdp enable
  5. end
  6. show running-config
  7. copy running-config startup-config

DETAILED STEPS

  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 global configuration mode.

Step 3

interface interface-id

Example:

Switch(config)# interface gigabitethernet1/0/1

Specifies the interface on which you are enabling Cisco Discovery Protocol, and enters interface configuration mode.

Step 4

cdp enable

Example:

Switch(config-if)# cdp enable

Enables Cisco Discovery Protocol 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 Cisco Discovery Protocol

Table 1. Commands for Displaying Cisco Discovery Protocol Information
Command Description

clear cdp counters

Resets the traffic counters to zero.

clear cdp table

Deletes the Cisco Discovery Protocol 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 Cisco Discovery Protocol 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 Cisco Discovery Protocol 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 Cisco Discovery Protocol counters, including the number of packets sent and received and checksum errors.