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2007.04.18   CDP(Cisco Discovery Protocol) Overview

CDP(Cisco Discovery Protocol) Overview

CDP overview


CDP (CISCO Discovery Protocol) is primarily used to obtain protocol addresses of neighboring devices and discover the platform of those devices. CDP can also be used to show 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 Management Information Base (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 runs on all media that support Subnetwork Access Protocol (SNAP), including local-area network (LAN), Frame Relay, and Asynchronous Transfer Mode (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.


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, which allows more rapid error tracking, thereby reduces costly downtime.


How CDP Works


All Cisco devices transmit CDP packets periodically (default time interval value is 60 seconds). These packets advertise a time-to-live value in seconds, which indicates the number of seconds that the packet must be retained before it can be discarded (default value is 180 seconds). CDP packets are sent with a time-to-live value that is nonzero after an interface is enabled and with a time-to-live value of zero immediately before an interface is idled down. This provides quick state discovery.

All Cisco devices receive CDP packets and cache the information in the packet. The cached information is available to network management. Cisco devices never forward a CDP packet. If any information changes from the last received packet, the new information is cached and the older information is discarded even if its time-to-live value has not yet expired.

CDP Frame Format


CDP is assigned the Cisco HDLC protocol type value 0x2000. A Cisco-proprietary SNAP value enumerates HDLC protocol type values so CDP can run on all media that support SNAP, such as LAN media, Frame Relay, and ATM.

The SNAP format is as follows:

  • LLC—0xAAAA03
  • Org ID—0x00000C
  • HDLC protocol type—0x2000

CDP sends packets on LANs using the multicast address 0100.0CCC.CCCC.


CDP packet structure


The CDP packet contains information about the Cisco devices in the network. It consists of a header, followed by a set of variable-length fields consisting of type/length/value triplets.



(1 byte)


(1 byte)


(2 bytes)


(2 bytes)


(2 bytes)


(variable length)





The version of CDP being used.


The amount of time, in seconds, that a receiver should retain the information contained in this packet.


The standard IP checksum.


The type/length/value type. The possible CDP type/length/value types are listed below.


The total length, in bytes, of the type, length, and value fields.


Contains the type/length/value value, which depends on the type/length/value type as described below.


Possible CDP type/length/value types


Definition of Value


Identifies the device name in the form of a character string. By default, the device ID is either the device's fully-qualified host name (including the domain name) or the device's hardware serial number in ASCII.


Contains a list of network addresses of both receiving and transmitting devices. The structure of this field will be shown below.


An ASCII character string that identifies the port on which the CDP message is sent


Describes the device's functional capability in the form of a device type. Field ‘Value’ length is constant – 4 bytes. Possible values are:




Performs level 3 routing for at least one network layer protocol.


Performs level 2 transparent bridging.


Performs level 2 source-route bridging. A source-route bridge would set both this bit and bit 0x02.


Performs level 2 switching. The difference between this bit and bit 0x02 is that a switch does not run the Spanning-Tree Protocol. This device is assumed to be deployed in a physical loop-free topology.


Sends and receives packets for at least one network layer protocol. If the device is routing the protocol, this bit should not be set.


The bridge or switch does not forward IGMP Report packets on nonrouter ports.


Provides level 1 functionality.


Contains information about the software release version on which the device is running – ASCII string.


An ASCII character string that describes the hardware platform of the device. The following are the possible string values:

  • Cisco 7000
  • Cisco 7010
  • Cisco 4500
  • Cisco 3100
  • Cisco 3000
  • Cisco 2500
  • Cisco 2000
  • Cisco 1000
  • AGS+
  • AGS
  • MGS
  • CGS
  • IGS
  • cs500
  • Catalyst
  • A100
  • Synergy

IP Network Prefix

Contains a set of 0 or more IP prefixes in its value field. No prefixes are included when the type/length/value's length field is 0. Otherwise, the length field includes the length of the type and value fields, plus 5 bytes for every IP prefix included. Each IP prefix consists of 4 bytes of IP network number and 1 byte representing the network mask. The network mask can be in the range 0 through 32, and represents the number of bits set in the mask (left contiguous).

  Each IP prefix represents one of the directly connected IP network segments of the local router. This type/length/value enables an IP stub router to communicate IP topology information to a central site router, without requiring the configuration of a full-blown IP routing protocol.

VTP Management Domain

Advertises a system's configured VTP management domain name-string. Used by network operators to verify VTP domain configuration in adjacent network nodes.

Native VLAN

Indicates, per interface, the assumed VLAN for untagged packets on the interface. CDP learns the native VLAN for an interface. This feature is implemented only for interfaces that support the IEEE 802.1Q protocol. Field ‘Value’ length is constant – 2 bytes.

Full/half Duplex

Indicates status (duplex configuration) of CDP broadcast interface. Used by network operators to diagnose connectivity problems between adjacent network elements. Field ‘Value’ length is constant – 1 byte.

Comment “*”: for triples of all types (except for ‘Address’) field ‘Length’ contains the length of the whole triple. In ‘Address’ type it contains the number of address records (addresses) in the current triple.

CDP address field structure

The address type/length/value ‘Length’ field contains a number that indicates how many addresses are contained in the packet, followed by one entry for each address being advertised. The addresses advertised are the ones assigned to the interface on which the CDP message is sent. A device can advertise all addresses for a given protocol suite and, optionally, can advertise one or more loopback IP addresses. If the device can be managed by SNMP, the first entry in the address type/length/value is an address at which the device receives SNMP messages.

The ‘Value’ field of the triple of this type is a sequence of the following records:


Protocol type
(1 byte)

(1 byte)


Address length
(2 bytes)





Protocol type

Can be one of the following values:

1—NLPID format

2—802.2 format


Length of the protocol field. For protocol type 1, the length is 1. For protocol type 2, the length is either 3 or 8, depending on whether SNAP is used.


One of the following values (X stands for hexadecimal digit):

  • 0x81—ISO CLNS (protocol type 3D 1)

hexadecimal number, any length above 14 and even is legal

  • 0xCC—IP (protocol type 3D 1)

XX . XX . XX . XX

  • 0xAAAA03 000000 0800—Pv6 (protocol type 3D 2)


  • 0xAAAA03 000000 6003—DECNET Phase IV (protocol type 3D 2)

 XX - XX - XX - XX - XX - XX

  • 0xAAAA03 000000 809B—AppleTalk (protocol type 3D 2)

XX . X . X

  • 0xAAAA03 000000 8137—Novell IPX (protocol type 3D 2)


  • 0xAAAA03 000000 80c4—Banyan VINES (protocol type 3D 2)


  • 0xAAAA03 000000 0600— XNS (protocol type 3D 2)


  • 0xAAAA03 000000 8019—Apollo Domain (protocol type 3D 2)


Address length

Length of the address field in bytes.


Address of the interface, or the address of the system if addresses are not assigned to the interface.


Protocol Structure - CDP: Cisco DiscoveryProtocol
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. The following table lists the CDP commands:



clear cdp counters

Resets the traffic counters to zero.

clear cdp table

Deletes the CDP table of information about neighbors.

show cdp

Displays the interval between transmissions of CDP advertisements, the number of seconds the CDP advertisement is valid for a given port, and the version of the advertisement.

show cdp entryentry-name [protocol| version]

Displays information about a specific neighbor. Display can be limited to protocol or version information.

show cdp interface[type number]

Displays information about interfaces on which CDP is enabled.

show cdp neighbors[type number] [detail]

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 port, the device type, the device product number, and the port ID. Issuing the detailkeyword displays information on the native VLAN ID, the duplex mode, and the VTP domain name associated with neighbor devices.

show cdp traffic

Displays CDP counters, including the number of packets sent and received and checksum errors.

show debugging

Displays information about the types of debugging that are enabled for your router. See the Cisco IOS Debug Command Reference for more information about CDP debugcommands.

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