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Table Of Contents
Configuring Cisco Discovery Protocol
Configuring the Cisco Discovery Protocol
Setting the CDP Transmission Timer and Hold Time
Reenabling CDP on a Local Router
Reenabling CDP Version-2 Advertisements
Reenabling CDP on an Interface
Monitoring and Maintaining CDP
Example: Setting the CDP Transmission Timer and Hold Time
Example: Monitoring and Maintaining CDP
Configuring Cisco Discovery Protocol
This chapter describes how the Cisco Discovery Protocol (CDP) works with Simple Network Management Protocol (SNMP) to identify other devices in your network in Cisco IOS Release 12.2.
For further details on the commands mentioned in this chapter, refer to the Cisco IOS Configuration Fundamentals Command Reference.
Configuring the Cisco Discovery Protocol
CDP is primarily used to obtain protocol addresses of neighboring devices and discover the platform of those devices. CDP can also be used to display information about the interfaces your router uses. CDP is media- and protocol-independent, and runs on all Cisco-manufactured equipment including routers, bridges, access servers, and switches.
Use of SNMP with the CDP MIB 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. CDP uses the CISCO-CDP-MIB.
CDP runs on all media that support Subnetwork Access Protocol (SNAP), including LAN, Frame Relay, and ATM physical media. CDP runs over the data link layer only. Therefore, two systems that support different network-layer protocols can learn about each other.
Each device configured for CDP sends periodic messages, known as advertisements, to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime, information, which indicates the length of time a receiving device should hold CDP information before discarding it. Each device also listens to the periodic CDP messages sent by others in order to learn about neighboring devices and determine when their interfaces to the media go up or down.
CDP Version-2 (CDPv2) is the most recent release of the protocol and provides more intelligent device tracking features. These features include a reporting mechanism that allows for more rapid error tracking, thereby reducing costly downtime. Reported error messages can be sent to the console or to a logging server, and cover instances of unmatching native VLAN IDs (IEEE 802.1Q) on connecting ports, and unmatching port duplex states between connecting devices. Refer tothe Cisco IOS Software System Error Messages document for detailed examples of CDP error messages.
CDPv2 show commands can provide detailed output on VLAN Trunking Protocol (VTP) management domain and duplex modes of neighbor devices, CDP-related counters, and VLAN IDs of connecting ports.
VTP is a discovery technique deployed by switches where each 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 one or more interconnected devices that share the same VTP domain name. A switch can be configured to be in only one VTP domain.
Type-Length-Value fields (TLVs) are blocks of information embedded in CDP advertisements. Table 19 summarizes the TLV definitions for CDP advertisements.
CDP Configuration Task List
To configure CDP, perform any of the optional tasks in the following sections:
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Setting the CDP Transmission Timer and Hold Time
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The the end of this chapter for "CDP Configuration Examples."
Note
Setting the CDP Transmission Timer and Hold Time
To set the frequency of CDP transmissions and the hold time for CDP packets, use the following commands in global configuration mode:
Reenabling CDP on a Local Router
CDP is enabled on Cisco devices by default. If you prefer not to use the CDP device discovery capability, you can disable it with the no cdp run command.
To reenable CDP after disabling it, use the following command in global configuration mode:
Reenabling CDP Version-2 Advertisements
The broadcasting of CDPv2 advertisements is enabled on Cisco routers by default. You can disable CDPv2 advertisements with the no cdp advertise-v2 command.
To reenable CDPv2 advertisements, use the following command in global configuration mode:
Router(config)# cdp advertise-v2
Enables CDPv2 advertising functionality on the system.
Reenabling CDP on an Interface
CDP is enabled by default on all supported interfaces (except for Frame Relay multipoint subinterfaces) to send and receive CDP information. However, some interfaces, such as ATM interfaces, do not support CDP.
You can disable CDP on an interface that supports CDP by using the no cdp enable command.
To reenable CDP on an interface after disabling it, use any of the following command in interface configuration mode, as needed:
Monitoring and Maintaining CDP
To monitor and maintain CDP on your device, use one or more of the following commands in privileged EXEC mode:
CDP Configuration Examples
The following sections provide CDP configuration examples:
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Example: Setting the CDP Transmission Timer and Hold Time
In the following example, the user sets the cdp timer to send updates every 30 seconds to neighboring routers and sets the router to show that the updates are working correctly:
Router(config)# cdp timer 30Router(config)# exitRouter# show cdp interfaceSerial0 is up, line protocol is upEncapsulation is HDLCSending CDP packets every 30 secondsHoldtime is 180 secondsIn the following example, the user sets the holdtime to be 90 seconds and sets the router to show that the updates are working correctly:
Router(config)# cdp holdtime 90Router(config)# exitRouter# show cdp interfaceSerial0 is up, line protocol is upEncapsulation is HDLCSending CDP packets every 30 secondsHoldtime is 90 secondsExample: Monitoring and Maintaining CDP
The following example shows a typical series of steps for viewing information about CDP neighbors.
Table 20 describes the significant fields shown in the output of the show cdp neighbors command.
C3660-2> show cdpGlobal CDP information:Sending CDP packets every 60 secondsSending a holdtime value of 180 secondsSending CDPv2 advertisements is enabledC3660-2> show cdp neighborsCapability Codes: R - Router, T - Trans Bridge, B - Source Route BridgeS - Switch, H - Host, I - IGMP, r - RepeaterDevice ID Local Intrfce Holdtme Capability Platform Port IDC2950-1 Fas 0/0 148 S I WS-C2950T-Fas 0/15RX-SWV.cisco.com Fas 0/1 167 T S WS-C3524-XFas 0/13C3660-2> show cdp neighbors detail-------------------------Device ID: C2950-1Entry address(es):Platform: Cisco WS-C2950T-24, Capabilities: Switch IGMPInterface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/15Holdtime : 139 secVersion :Cisco IOS C2950 Software (C2950-I6Q4L2-M), Version 12.1(9)EA1, RELEASE SOFTWARE...C3660-2> show cdp trafficCDP counters :Total packets output: 81684, Input: 81790Hdr syntax: 0, Chksum error: 0, Encaps failed: 0No memory: 0, Invalid packet: 0, Fragmented: 0CDP version 1 advertisements output: 0, Input: 0CDP version 2 advertisements output: 81684, Input: 81790C3660-2>Table 20 describes the significant fields shown in the output of the show cdp neighbors command.
Table 22 show cdp neighbors Field Descriptions
