BACK
Cisco Discovery Protocol
Understanding How CDP Works
CDP is a media and protocol independent protocol that runs on all Cisco-manufactured equipment including
routers, bridges, access and communication servers, and switches. Using CDP, you can view information about all the
Cisco devices directly attached to the switch. In addition, CDP detects native VLAN and port duplex mismatches.
Network management applications can retrieve the device type and SNMP-agent address of neighboring Cisco
devices using CDP. This enables applications to send SNMP queries to neighboring devices. CDP allows network
management applications to discover Cisco devices that are neighbors of already known devices, in particular,
neighbors running lower-layer, transparent protocols.
CDP runs on all media that support Subnetwork Access Protocol (SNAP), including LAN and Frame Relay. CDP
runs over the Data Link layer only. Cisco devices never forward CDP packets. When new CDP information is received,
Cisco devices discard old information.
CDP Default Configuration
Feature
Default Value
CDP global enable state
Enabled
CDP port enable state
Enabled on all ports
CDP message interval
60 seconds
CDP holdtime
180 seconds
Getting CDP Timer and Holdtime Information
The following command shows the CDP timer which is how often CDP packets are sent and the CDP holdtime
which is the amount of time that the device will hold packets from neighbor devices.
Router_2#sho cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Setting the Holdtime and Timer
Use the following commands to set CDP timer and holdtime values.
Router_2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router_2(config)#cdp timer 90
Router_2(config)#cdp holdtime 360
CDP can be disabled with the no cdp run command in global configuration mode (conf t).
Getting Neighbor Information
Because the router stores the CDP information in its cache memory, you can view it with a show command. It will
only show information about directly connected devices since CDP packet aren't passed through the device.
Router_2#sho cdp neighbors
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater
Device ID
Router3
Router1
Switch1
Local Intrfce
Ser 1
Eth 1
Eth 0
Holdtme
120
180
240
Capability
R
R
S
Platform
2500
2500
1900
Port ID
Ser 0
Eth 0
2
CDP Neighbor Information includes
Neighbor's device ID
Local port type and number
converted by Web2PDFConvert.com
Holdtime value (in seconds)
Neighbor's network device capability
Neighbor's hardware platform
Neighbor's remote port type and number
Show CDP Entry
The sho cdp entry [device id] command shows more information about the specified neighbor.
Router_2#sho cdp entry Router1
------------------------Device ID: Router1
Entry address(es):
IP address: 192.168.1.2
Platform: cisco 2500, Capabilities: Router
Interface: Ethernet1, Port ID (outgoing port): Ethernet0
Holdtime : 180 sec
Version:
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (2500-JS-L), Version 11.2(15)
RELEASED SOFTWARE (fcl)
Copyright (c) 1986-1998 by Cisco Systems, Inc.
Compiled Mon 06-Jul-98 22:22 by tmullins
Show CDP Neighbors Detail
The following is a sample output for one neighbor from the show cdp neighbors detail command. Additional
detail is shown about neighbors, including network address, enabled protocols, and software version.
router#show cdp neighbors detail
Device ID: 008024 1EEB00 (milan-sw-1-cat9k)
Entry address(es):
IP address: 1.15.28.10
Platform: CAT5000, Capabilities: Switch
Interface: Ethernet1/0, Port ID (outgoing port): 2/7
Holdtime : 162 sec
Version :
Cisco Catalyst 5000
Duplex Mode: full
Native VLAN: 42
VTP Management Domain: `Accounting Group'
show cdp neighbors detail Field Descriptions
Field
Definition
Device ID
The name of the neighbor device and either the MAC address or the serial number of this device.
Entry
address(es)
A list of network addresses of neighbor devices.
[network
protocol]
address
The network address of the neighbor device. The address can be in IP, IPX, AppleTalk, DECnet, or
CLNS protocol conventions.
Platform
The product name and number of the neighbor device.
Capabilities
The device type of the neighbor. This device can be a router, a bridge, a transparent bridge, a
source-routing bridge, a switch, a host, an IGMP device, or a repeater.
Interface
The protocol and port number of the port on the current device.
Holdtime
The remaining amount of time, in seconds, the current device will hold the CDP advertisement from
a transmitting router before discarding it.
Version
The software version of the neighbor device.
Duplex Mode
The duplex state of connection between the current device and the neighbor device.
Native VLAN
The ID number of the VLAN on the neighbor device.
VTP
Management
Domain
A string that is the name of the collective group of VLANs associated with the neighbor device.
Show CDP Traffic
The following example specifies information associated with the show cdp traffic command:
router# show cdp traffic
converted by Web2PDFConvert.com
Total packets output: 543, Input: 333
Hdr syntax: 0, Chksum error: 0, Encaps failed: 0
No memory: 0, Invalid: 0, Fragmented: 0
CDP version 1 advertisements output: 191, Input: 187
CDP version 2 advertisements output: 352, Input: 146
show cdp traffic Fields
Field
Definition
Total packets
output
The number of CDP advertisements transmitted by the local device. Note this value is the sum of
the CDP Version-1 advertisements output and CDP Version-2 advertisements output fields.
Input
The number of CDP advertisements received by the local device. Note this value is the sum of the
CDP Version-1 advertisements input and CDP Version-2 advertisements input fields.
Hdr syntax
The number of CDP advertisements with bad headers, received by the local device.
Chksum error
The number of times the checksum (verifying) operation failed on incoming CDP advertisements.
Encaps failed
The number of times CDP failed to transmit advertisements on an interface because of a failure
caused by the local device's bridge port.
No memory
The number of times the local device did not have enough memory to store the CDP advertisements
in the advertisement cache table when the device was attempting to assemble advertisement
packets for transmission and parse them when receiving them.
Invalid
The number of invalid CDP advertisements received and transmitted by the local device.
Fragmented
The number of times fragments or portions of a single CDP advertisement were received by the local
device instead of the complete advertisement.
CDP version 1 The number of CDP Version-1 advertisements transmitted by the local device.
advertisements
output
Input
The number of CDP Version-1 advertisements received by the local device.
CDP version 2 The number of CDP Version-2 advertisements transmitted by the local device.
advertisements
output
Input
The number of CDP Version-2 advertisements received by the local device.
Show CDP Interface
To display information about the interfaces on which CDP is enabled, use the show cdp interface privileged
EXEC command.
Router#sh cdp interface [type number]
type
(Optional) Type of interface about which you want information.
number
(Optional) Number of the interface about which you want information.
Router_2#sho cdp interface
Ethernet0 is up, line protocol is up
Encapsulation ARPA
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial0 is up, line protocol is up
Encapsulation HDLC
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Serial1 is up, line protocol is up
Encapsulation HDLC
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
BACK
converted by Web2PDFConvert.com