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 Local Intrfce Holdtme Capability Platform Port ID
Router3 Ser 1 120 R 2500 Ser 0
Router1 Eth 1 180 R 2500 Eth 0
Switch1 Eth 0 240 S 1900 2
CDP Neighbor Information includes
- Neighbor's device ID
- Local port type and number
- 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
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 advertisements output |
The number of CDP Version-1 advertisements transmitted by the
local device. |
| Input
|
The number of CDP Version-1 advertisements received by the
local device. |
| CDP version 2 advertisements output |
The number of CDP Version-2 advertisements transmitted by the
local device. |
| 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
|