Catalyst 3550 Multilayer Switch Software Configuration Guide, 12.1(8)EA1
Configuring CDP
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Configuring CDP

Table Of Contents

Configuring CDP

Understanding CDP

Configuring CDP

Default CDP Configuration

Configuring the CDP Characteristics

Disabling and Enabling CDP

Disabling and Enabling CDP on an Interface

Monitoring and Maintaining CDP


Configuring CDP


This chapter describes how to configure Cisco Discovery Protocol (CDP) on your switch.


Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 3550 Multilayer Switch Command Reference for this release and the Cisco IOS Configuration Fundamentals Command Reference for Release 12.1.


This chapter consists of these sections:

Understanding CDP

Configuring CDP

Monitoring and Maintaining CDP

Understanding CDP

CDP is a device discovery protocol that runs over Layer 2 (the data link layer) on all Cisco-manufactured devices (routers, bridges, access servers, and switches) and allows network management applications to discover Cisco devices that are neighbors of already known devices. With CDP, network management applications can learn the device type and the Simple Network Management Protocol (SNMP) agent address of neighboring devices running lower-layer, transparent protocols. This feature enables applications to send SNMP queries to neighboring devices.

CDP runs on all media that support Subnetwork Access Protocol (SNAP). Because CDP runs over the data-link layer only, two systems that support different network-layer protocols can learn about each other.

Each CDP-configured device sends periodic messages to a multicast address, advertising at least one address at which it can receive SNMP messages. The advertisements also contain time-to-live, or holdtime information, which is the length of time a receiving device holds CDP information before discarding it. Each device also listens to the messages sent by other devices to learn about neighboring devices.

On the Catalyst 3550 switch, CDP enables the Cluster Management Suite to display a graphical view of the network. The switch uses CDP to find cluster candidates and maintain information about cluster members and other devices up to three cluster-enabled devices away from the command switch by default.

The switch supports CDP version 2.

Configuring CDP

These sections include CDP configuration information and procedures:

Default CDP Configuration

Configuring the CDP Characteristics

Disabling and Enabling CDP

Disabling and Enabling CDP on an Interface

Default CDP Configuration

Table 13-1 shows the default CDP configuration.

Table 13-1 Default CDP Configuration

Feature
Default Setting

CDP global state

Enabled

CDP interface state

Enabled

CDP timer (packet update frequency)

60 seconds

CDP holdtime (before discarding)

180 seconds

CDP version-2 advertisements

Enabled


Configuring the CDP Characteristics

You can configure the frequency of CDP updates, the amount of time to hold the information before discarding it, and whether or not to send version-2 advertisements.

Beginning in privileged EXEC mode, follow these steps to configure the CDP timer, holdtime, and advertisement type.


Note Steps 2 through 4 are all optional and can be performed in any order.


 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

cdp timer seconds

(Optional) Set the transmission frequency of CDP updates in seconds.

The range is from 5 to 254; the default is 60 seconds.

Step 3 

cdp holdtime seconds

(Optional) Specify the amount of time a receiving device should hold the information sent by your device before discarding it.

The range is from 10 to 255 seconds; the default is 180 seconds.

Step 4 

cdp advertise-v2

(Optional) Configure CDP to send version-2 advertisements.

This is the default state.

Step 5 

end

Return to privileged EXEC mode.

Step 6 

show cdp

Verify configuration by displaying global information about CDP on the device.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Use the no form of the CDP commands to return to the default settings.

This example shows how to configure and verify CDP characteristics.

Switch# configure terminal
Switch(config)# cdp timer 50
Switch(config)# cdp holdtime 120
Switch(config)# cdp advertise-v2
Switch(config)# end

Switch# show cdp

Global CDP information:
        Sending CDP packets every 50 seconds
        Sending a holdtime value of 120 seconds
        Sending CDPv2 advertisements is enabled 

For additional CDP show commands, see the "Monitoring and Maintaining CDP" section.

Disabling and Enabling CDP

CDP is enabled by default.


Note Creating and maintaining switch clusters is based on the regular exchange of CDP messages. Disabling CDP can interrupt cluster discovery. For more information, see "Clustering Switches."


Beginning in privileged EXEC mode, follow these steps to disable the CDP device discovery capability:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

no cdp run

Disable CDP.

Step 3 

end

Return to privileged EXEC mode.

Beginning in privileged EXEC mode, follow these steps to enable CDP when it has been disabled:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

cdp run

Enable CDP after disabling it.

Step 3 

end

Return to privileged EXEC mode.

This example shows how to enable CDP if it has been disabled.

Switch# configure terminal
Switch(config)# cdp run
Switch(config)# end

Disabling and Enabling CDP on an Interface

CDP is enabled by default on all supported interfaces to send and receive CDP information.

Beginning in privileged EXEC mode, follow these steps to disable CDP on an interface:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and enter the interface on which you are disabling CDP.

Step 3 

no cdp enable

Disable CDP on an interface.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

Beginning in privileged EXEC mode, follow these steps to enable CDP on an interface when it has been disabled:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and enter the interface on which you are enabling CDP.

Step 3 

cdp enable

Enable CDP on an interface after disabling it.

Step 4 

end

Return to privileged EXEC mode.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

This example shows how to enable CDP on an interface when it has been disabled.

Switch# configure terminal
Switch(config)# interface gigabitethernet0/5
Switch(config-if)# cdp enable
Switch(config-if)# end

Monitoring and Maintaining CDP

To monitor and maintain CDP on your device, perform one or more of these tasks, beginning in privileged EXEC mode.

Command
Description

clear cdp counters

Reset the traffic counters to zero.

clear cdp table

Delete the CDP table of information about neighbors.

show cdp

Display global information, such as frequency of transmissions and the holdtime for packets being sent.

show cdp entry entry-name [protocol | version]

Display information about a specific neighbor.

You can enter an asterisk (*) to display all CDP neighbors, or you can enter the name of the neighbor about which you want information.

You can also limit the display to information about the protocols enabled on the specified neighbor or information about the version of software running on the device.

show cdp interface [type number]

Display information about interfaces where CDP is enabled.

You can limit the display to the type of interface or the number of the interface about which you want information (for example, entering gigabitethernet 0/1 displays information only about Gigabit Ethernet port 1).

show cdp neighbors [type number] [detail]

Display information about neighbors, including device type, interface type and number, holdtime settings, capabilities, platform, and port ID.

You can limit the display to neighbors on a specific type or number of interface or expand the display to provide more detailed information.

show cdp traffic

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

These are examples of outputs from the CDP show privileged EXEC commands:

Switch# show cdp

Global CDP information:
        Sending CDP packets every 50 seconds
        Sending a holdtime value of 120 seconds
        Sending CDPv2 advertisements is enabled

Switch# show cdp entry *
-------------------------
Device ID: Switch
Entry address(es):
  IP address: 10.1.1.66
Platform: cisco WS-C3550-12T,  Capabilities: Switch IGMP
Interface: GigabitEthernet0/2,  Port ID (outgoing port): GigabitEthernet0/2
Holdtime : 129 sec

Version :
Cisco Internetwork Operating System Software
IOS (tm) C3550 Software (C3550-I5Q3L2-M), Experimental Version 12.1(20010612:021
316) [jang-flamingo 120]
Copyright (c) 1986-2001 by cisco Systems, Inc.
Compiled Fri 06-Jul-01 18:18 by jang

advertisement version: 2
Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=27, value=0000000
0FFFFFFFF010221FF00000000000000024B293A00FF0000
VTP Management Domain: ''
Duplex: full

-------------------------
Device ID: idf2-1-lab-l3.cisco.com
Entry address(es):
  IP address: 10.1.1.10
Platform: cisco WS-C3524-XL,  Capabilities: Trans-Bridge Switch
Interface: GigabitEthernet0/1,  Port ID (outgoing port): FastEthernet0/10
Holdtime : 141 sec

Version :
Cisco Internetwork Operating System Software
IOS (tm) C3500XL Software (C3500XL-C3H2S-M), Version 12.0(5.1)XP, MAINTENANCE IN
TERIM SOFTWARE
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Fri 10-Dec-99 11:16 by cchang

advertisement version: 2
Protocol Hello:  OUI=0x00000C, Protocol ID=0x0112; payload len=25, value=0000000
0FFFFFFFF010101FF000000000000000142EFA400FF
VTP Management Domain: '' 

Switch# show cdp entry * protocol
Protocol information for talSwitch14 :
  IP address: 172.20.135.194
Protocol information for tstswitch2 :
  IP address: 172.20.135.204
  IP address: 172.20.135.202
Protocol information for tstswitch2 :
  IP address: 172.20.135.204
  IP address: 172.20.135.202 

Switch# show cdp interface
GigabitEthernet0/1 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/2 is up, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/3 is administratively down, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/4 is up, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/5 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/6 is up, line protocol is up
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/7 is up, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
GigabitEthernet0/8 is up, line protocol is down
  Encapsulation ARPA
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds 

Switch# show cdp neighbor
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
Perdido2	Gig 0/6	125	R S I	WS-C3550-1Gig				0/6
Perdido2	Gig 0/5	125	R S I	WS-C3550-1Gig 	0/5 

Switch# show cdp traffic
CDP counters :
        Total packets output: 50882, Input: 52510
        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: 50882, Input: 52510