Cisco Discovery Protocol Configuration Guide, Cisco IOS Release 12.4T
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- Updated:
- October 15, 2011
Chapter: Using Cisco Discovery Protocol
- Finding Feature Information
- Prerequisites for Using CDP
- Restrictions for Using CDP
- Information About Using CDP
- How to Use CDP
- Configuration Examples for Using CDP
- Additional References
- Feature Information for Using CDP
Using Cisco Discovery Protocol
Cisco Discovery Protocol (CDP) is a Layer 2 media-independent and network-independent protocol that runs on Cisco devices and enables networking applications to "learn" about nearby, directly connected devices. 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 one another.
This module describes CDP and how it functions with Simple Network Management Protocol (SNMP).
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the Feature Information Table at the end of this document.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Prerequisites for Using CDP
- Interfaces must support Subnetwork Access Protocol (SNAP) headers.
Restrictions for Using CDP
- CDP functions only on Cisco devices.
- CDP is not supported on Frame Relay multipoint subinterfaces.
Information About Using CDP
VLAN Trunking Protocol
VLAN Trunking Protocol (VTP) is a discovery technique that is used by switches. A switch advertises its management domain on its trunk ports, its configuration revision number, and its known VLANs and their specific parameters. A VTP domain is made up of a single device or interconnected devices that share the same VTP domain name. A switch can be in only one VTP domain.
Type-Length-Value Fields
Type-Length-Value (TLV) fields are blocks of information embedded in CDP advertisements. Information in advertisements varies, and the TLV frame format allows for extending advertisements when needed. The table below summarizes the TLV definitions.
| Table 1 | Type-Length-Value Definitions for CDP v2 |
CDP
CDP is a Layer 2 media-independent and network-independent protocol that networking applications use to learn about nearby, directly connected devices. It is enabled by default. Each device configured for CDP advertises at least one address at which it can receive messages and sends periodic advertisements (messages) to the well-known multicast address 01:00:0C:CC:CC:CC. Devices discover each other by listening at that address. They also listen to messages to learn when interfaces on the other devices are up or go down.
Advertisements contain time-to-live information, which indicates the length of time a receiving device should hold CDP information before discarding it. Advertisements supported and configured in the Cisco IOS software are sent by default every 60 seconds on interfaces that support SNAP headers. Cisco devices never forward CDP packets. Each Cisco device that supports CDP stores the information it receives in a table. Information in the table is refreshed each time an advertisement is received, and information about a device is discarded after three advertisements from that device are missed.
The information contained in CDP advertisements varies by the type of device and the version of the operating system running on it. Following is some of the information that CDP can learn:
- Cisco IOS version running on a Cisco device
- Hardware platform of the device
- IP addresses of the interfaces on devices
- Locally connected devices advertising CDP
- Interfaces active on a Cisco device, including encapsulation type
- Hostname
- Duplex setting
- VTP domain
- Native VLAN
CDP version 2 provides more intelligent device tracking features than those available in version 1. One feature is an enhanced reporting mechanism for more rapid error tracking, which helps to reduce network downtime. Errors reported include mismatched native VLAN IDs (IEEE 802.1Q) on connected ports and mismatched port duplex states between connected devices. Messages about reported errors can be sent to the console or to a logging server.
Cisco IOS CDP show commands can provide detailed output on VTP management domains and duplex modes of neighboring devices, counters related to CDP, and VLAN IDs of connecting ports.
- Using CDP with SNMP
- CDP and On-Demand Routing Support for ATM PVCs
- CDP Support in IPv6
- CDP Enhancements for Location Support
Using CDP with SNMP
Using CDP with SNMP allows network management applications to learn the device type and the SNMP agent address of neighboring devices and to send SNMP queries to those devices.
The SNMP management application learns protocol addresses and types of neighboring devices by retrieving the CDP tables from SNMP agents in those devices. When enabled, the network management module (NMM) SNMP agent discovers neighboring devices and builds its local cache with information about these devices. A management workstation can retrieve this cache by sending SNMP requests to access the CISCO-CDP-MIB.
CDP and On-Demand Routing Support for ATM PVCs
In Cisco IOS Release 12.2(8)T CDP and On-Demand Routing (ODR) support was added for ATM point-to-point permanent virtual circuits (PVCs). ODR uses CDP to propagate IP address information in hub-and-spoke topologies. When ODR is enabled, spoke routers automatically advertise their subnets using CDP.
CDP is disabled by default on ATM PVC interfaces. To enable CDP, use the cdp run command in global configuration mode and the cdp enable command in interface configuration mode command on both ends of the PVC. To enable ODR, use the router odr command in global configuration mode on the hub router and turn off all dynamic routing protocols in the spoke routers. For details on configuring ODR, see http://www.cisco.com/en/US/docs/ios/iproute_odr/configuration/guide/ird_odrconfg.html.
CDP Support in IPv6
CDP in IPv6 functions the same as IPv4 and offers the same benefits as in IPv4. IPv6 enhancements allow CDP to exchange IPv6 and neighbor addressing information, provide IPv6 information to network management products, and provide troubleshooting tools.
CDP Enhancements for Location Support
The CDP Medianet Support feature allows CDP to identify the endpoint devices and provides the ability to exchange the medianet location information between endpoint and switch. Location information can be provisioned statically by the user on the platform, or can be received through updates from the location server, or can be received from the endpoint itself through CDP.
The Propagation of Location Info over CDP feature ensures location information to be transmitted from wired switches over CDP to the attached end devices.
CDP allows only one wired switch to report the location information by identifying and indicating the location of neighbor ports not to report the duplicate location information of the same device.
For more information on location TLVs, refer to Using Link Layer Discovery Protocol in Multivendor Networks.
Benefits of CDP
CDP provides the following benefits:
- Allows systems using different network-layer protocols to learn about one another.
- Facilitates managing Cisco devices by discovering them and how they are configured.
- Assists with troubleshooting TLV fields.
- Functions with SNMP to learn SNMP agent addresses and to send SNMP queries.
How to Use CDP
- Disabling and Enabling CDP on a Supported Device
- Disabling and Enabling CDP on a Supported Interface
- Setting the Transmission Timer and Hold Time
- Disabling and Reenabling CDP Version 2 Advertisements
- Configuring Location Support on CDP
- Monitoring and Maintaining CDP
Disabling and Enabling CDP on a Supported Device
Perform the following tasks to disable and enable CDP on a supported device:
Disabling CDP on a Supported Device
Perform this task to disable CDP on a Cisco device.
DETAILED STEPS
Enabling CDP on a Supported Device
Perform this task to enable CDP on a Cisco device.
DETAILED STEPS
Disabling and Enabling CDP on a Supported Interface
CDP is enabled by default on supported interfaces on a Cisco device. To disable or reenable CDP on an interface, perform these tasks:
Disabling CDP on a Supported Interface
Perform this task to disable CDP on a supported interface.
 Note |
If the encapsulation for an interface is changed, it reenables CDP on that interface, even if CDP is previously disabled. For example, when interface encapsulation changes from PPP to HDLC, CDP becomes active even if it is explicitly disabled with the no cdp run command on that interface. This behavior is by design. The encapsulation changes the Layer 2 protocol configured for that interface and resets the interface configuration to its default CDP state of enabled, assuming CDP is enabled globally on the device. |
DETAILED STEPS
Example
If the encapsulation for an interface is changed, it reenables CDP on that interface, even if CDP is previously disabled. For example, when the interface encapsulation changes from PPP to HDLC, CDP become active even if it is explicitly disabled with the no cdp run command on that interface. This behavior is by design. The encapsulation changes the Layer 2 protocol configured for that interface and resets the interface configuration to its default CDP state of enabled, assuming CDP is enabled globally on the device.
The following examples show that when the encapsulation for an interface is changed, it reenables CDP on that interface, even if CDP is previously disabled:
interface POS3/0 no ip address encapsulation ppp shutdown no cdp enable <<<<<<<<<<CDP Disabled end
The following example shows that after CDP is disabled, the encapsulation for the interface is changed from PPP to HDLC:
Router# configure terminal Enter configuration commands, one per line. End with CNTL/Z. Router(config)# interface fastethernet 3/0 Router(config-if)# encap hdlc <<<<<<<<<<<<Encapsulation changed from ppp to hdlc. Router(config-if)# do show running interface fastethernet 3/0 Current configuration : 49 bytes !
The following example shows that CDP is reenabled after the encapsulation for the interface is changed.
interface POS3/0 no ip address shutdown <<<<<<<<<<<<CDP is now back on. You don't see it here previously disabled. end
Enabling CDP on a Supported Interface
Perform this task to enable CDP on a supported interface.
Note |
If the encapsulation for an interface is changed, it reenables CDP on that interface, even if CDP is previously disabled. |
DETAILED STEPS
| Command or Action | Purpose | |||
|---|---|---|---|---|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# interface ethernet 0/1 |
Configures the specified interface and places the CLI in interface configuration mode.
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Example: Router(config-if)# cdp enable |
Enables CDP on the interface. |
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Example: Router(config-if)# end |
Returns the CLI to privileged EXEC mode. |
Setting the Transmission Timer and Hold Time
Perform this task to set the frequency of CDP transmissions and the hold time for CDP packets.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# cdp timer 30 |
Specifies the frequency of transmission of CDP packets. |
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Example: Router(config)# cdp holdtime 90 |
Specifies the amount of time a receiving device should hold information before discarding it. |
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Example: Router(config)# end |
Returns the CLI to privileged EXEC mode. |
Disabling and Reenabling CDP Version 2 Advertisements
The broadcasting of CDP version 2 advertisements is enabled by default on Cisco devices. To disable or reenable this broadcasting, perform these tasks.
Disabling CDP Version 2 Advertisements
Perform this task to disable CDP version 2 advertisements.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# no cdp advertise-v2 |
Disables the broadcasting of CDP version 2 advertisements. |
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Example: Router(config)# end |
Returns the CLI to privileged EXEC mode. |
Enabling CDP Version 2 Advertisements
Perform this task to enable CDP version 2 advertisements.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# cdp advertise-v2 |
Enables the broadcasting of CDP version 2 advertisements. |
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Example: Router(config)# end |
Returns the CLI to privileged EXEC mode. |
Configuring Location Support on CDP
Perform this task to configure location support on CDP.
You must configure the location TLV before configuring location support on CDP. For more information on configuring location TLV, refer to Using Link Layer Discovery Protocol in Multivendor Networks.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
|
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Example: Router> enable |
Enables privileged EXEC mode. |
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Example: Router# configure terminal |
Enters global configuration mode. |
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Example: Router(config)# cdp tlv app |
Configures location support in CDP. |
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Example: Router(config-civic)# exit |
Returns the CLI to global configuration mode. |
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Example: Router(config)# interface fastethernet 0 |
Specifies the interface on which you are configuring the location support for CDP, and enters interface configuration mode. |
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Example: Router(config-if)# cdp tlv tlvtype 20 value test12 |
Configures location support in CDP on a specified interface. |
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Example: Router(config-if-port)# end |
Returns the CLI to privileged EXEC mode. |
Monitoring and Maintaining CDP
To monitor and maintain CDP on your device, perform the following steps. This task and all steps are optional, and you do not have to perform the steps in the sequence shown.
DETAILED STEPS
| Command or Action | Purpose | |
|---|---|---|
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Example: Router> enable |
Enables privileged EXEC mode.
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Example: Router# clear cdp counters |
Resets the traffic counters to zero. |
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Example: Router# clear cdp table |
Deletes the CDP table of information about neighbors. |
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Example: Router# show cdp |
Displays the interval between advertisements, the number of seconds the advertisement is valid for a given port, and the version of the advertisement. |
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Example: Router# show cdp entry test-device protocol |
Displays information about a specific neighbor.
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Example: Router# show cdp interface |
Displays information about interfaces on which CDP is enabled. |
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Example: Router# show cdp neighbors |
Displays the type of device that has been discovered, the name of the device, the number and type of the local interface (port), the number of seconds the CDP advertisement is valid for the interface, the device type, the device product number, and the port ID. Issuing the detail keyword displays information about the native VLAN ID, the duplex mode, and the VTP domain name associated with neighbor devices. |
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Example: Router# show cdp traffic |
Displays CDP counters, including the number of packets sent and received and checksum errors. |
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Example: Router# show debugging |
Displays information about the types of debugging that are enabled for the router. |
Configuration Examples for Using CDP
The following examples show how to set transmission timer and hold time values and show commands you can issue to monitor and maintain CDP.
Example Setting the Transmission Timer and Hold Time
In the following example, the timer is set to send updates every 30 seconds and a show cdp interface command is issued to show that the update is effective:
Router(config)# cdp timer 30 Router(config)# end Router# show cdp interface Serial0 is up, line protocol is up Encapsulation is HDLC Sending CDP packets every 30 seconds Holdtime is 180 seconds
In the following example, the hold time is set to 90 seconds and a show cdp interface command is issued to show that the update is effective:
Router(config)# cdp holdtime 90 Router(config)# end Router# show cdp interface Serial0 is up, line protocol is up Encapsulation is HDLC Sending CDP packets every 30 seconds Holdtime is 90 seconds
Example Monitoring and Maintaining CDP
The following example shows a typical series of commands you can issue to view CDP information:
Router# show cdp Global CDP information: Sending CDP packets every 60 seconds Sending a holdtime value of 180 seconds Sending CDPv2 advertisements is enabled Router# 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 Interfce Holdtme Capability Platform Port ID C2950-1 Fas 0/0 148 S I WS-C2950T-Fas 0/15 RX-SWV.cisco.com Fas 0/1 167 T S WS-C3524-XFas 0/13 Router# show cdp neighbors detail ------------------------- Device ID: C2950-1 Entry address(es): Platform: Cisco WS-C2950T-24, Capabilities: Switch IGMP Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/15 Holdtime : 139 sec Version : Cisco IOS C2950 Software (C2950-I6Q4L2-M), Version 12.1(9)EA1, RELEASE SOFTWARE . . . Router# show cdp traffic CDP counters : Total packets output: 81684, Input: 81790 Hdr syntax: 0, Chksum error: 0, Encaps failed: 0 No memory: 0, Invalid packet: 0, Fragmented: 0 CDP version 1 advertisements output: 0, Input: 0 CDP version 2 advertisements output: 81684, Input: 81790
Additional References
Related Documents
| Related Topic |
Document Title |
|---|---|
| Cisco IOS CDP commands |
Cisco IOS Network Management Command Reference |
| Configuring SNMP Support |
"Configuring SNMP Support" module |
| Debugging commands |
Cisco IOS Debug Command Reference |
| Configuring On-Demand Routing |
"Configuring On-Demand Routing" module |
| Configuring Location TLVs |
"Using Link Layer Discovery Protocol in Multivendor Networks" module |
Standards
| Standard |
Title |
|---|---|
| IEEE 802.1Q |
Virtual LANS |
MIBs
| MIB |
MIBs Link |
|---|---|
| To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
RFCs
| RFC |
Title |
|---|---|
| No new or modified RFCs are supported, and support for existing RFCs has not been modified. |
-- |
Technical Assistance
| 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. |
Feature Information for Using CDP
The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
| Table 2 | Feature Information for Configuring CDP |
| Feature Name |
Releases |
Feature Information |
|---|---|---|
| CDP Medianet Support |
12.2(55)SE |
The CDP Medianet Support feature allows CDP to identify the endpoint devices and provides the ability to exchange the medianet location information between endpoint and switch. |
| Configuring CDP |
10.3, 11.2(1) 12.2(8)T |
CDP is a Layer 2 media-independent and network-independent protocol that runs on Cisco devices and enables networking applications to "learn" about nearby, directly connected devices. This protocol facilitates the management of Cisco devices and allows systems using different network-layer protocols to learn about one another. |
| Propagation of Location Info over CDP |
12.2(55)SE |
The Propagation of Location Info over CDP ensures location information to be transmitted from wired switches over CDP to the attached end devices. The following commands were introduced or modified: cdp tlv, show cdp tlv, debug cdp events. |
Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)
Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.
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