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Information About 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, 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.
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.
This table shows the default CDP configuration.
Feature | Default Setting |
---|---|
CDP global state | Enabled |
CDP interface state | Enabled |
CDP timer (packet update frequency) | 60 seconds |
CDP holdtime (before discarding) | 180 seconds |
CDP Version-2 advertisements | Enabled |
How to Configure CDP
![]() Note | Steps 3 through 5 are all optional and can be performed in any order. |
Follow these steps to configure the CDP characteristics.
1.
enable
3.
cdp timer
seconds
4.
cdp holdtime
seconds
5.
cdp advertise-v2
8.
copy running-config
startup-config
Use the no form of the CDP commands to return to the default settings.
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. |
Follow these steps to disable the CDP device discovery capability.
1.
enable
3.
no cdp run
6.
copy running-config
startup-config
You must reenable CDP to use it.
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. |
Follow these steps to enable CDP when it has been disabled.
CDP must be disabled, or it cannot be enabled.
1.
enable
3.
cdp run
6.
copy running-config
startup-config
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.
CDP is enabled by default on all supported interfaces to send and to receive CDP information.
![]() 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.
1.
enable
3.
interface
interface-id
4.
no cdp enable
7.
copy running-config
startup-config
CDP is enabled by default on all supported interfaces to send and to receive CDP information.
![]() 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.
CDP must be disabled on the port that you are trying to CDP enable on, or it cannot be enabled.
1.
enable
3.
interface
interface-id
4.
cdp enable
7.
copy running-config
startup-config
Command | Description |
---|---|
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. |
Related Topic | Document Title |
---|---|
System Management Commands |
Network Management Command Reference, Cisco IOS Release 15.2(2)E |
Description | Link |
---|---|
To help you research and resolve system error messages in this release, use the Error Message Decoder tool. |
https://www.cisco.com/cgi-bin/Support/Errordecoder/index.cgi |
Standard/RFC | Title |
---|---|
None |
- |
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: |
Description | Link |
---|---|
The Cisco Support website provides extensive online resources, including documentation and tools for troubleshooting and resolving technical issues with Cisco products and technologies. To receive security and technical information about your products, you can subscribe to various services, such as the Product Alert Tool (accessed from Field Notices), the Cisco Technical Services Newsletter, and Really Simple Syndication (RSS) Feeds. Access to most tools on the Cisco Support website requires a Cisco.com user ID and password. |
Release |
Modification |
---|---|
Cisco IOS 15.0(2)EX |
This feature was introduced. |