Catalyst 6500 Series Software Configuration Guide, 8.7
Configuring CDP
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Table Of Contents

Configuring CDP

Understanding How CDP Works

Default CDP Configuration

Configuring CDP on the Switch

Setting the CDP Global Enable and Disable States

Setting the CDP Enable and Disable States on a Port

Setting the CDP Message Interval

Setting the CDP Holdtime

Displaying CDP Neighbor Information


Configuring CDP

This chapter describes how to configure the Cisco Discovery Protocol (CDP) on the Catalyst 6500 series switches.

Note For complete syntax and usage information for the commands that are used in this chapter, refer to the Catalyst 6500 Series Switch Command Reference publication.

This chapter consists of these sections:

•Understanding How CDP Works

•Default CDP Configuration

•Configuring CDP on the Switch

Understanding How CDP Works

CDP was enhanced in software release 8.1(1) to facilitate the backward compatibility with the newer, higher-powered Cisco IP phones. With this enhanced CDP, a Cisco IP phone can negotiate its power requirements to the switch within the CDP packet. The switch uses this information to ensure that it does not oversubscribe the available power.

CDP is a media- and protocol-independent protocol that runs on all the Cisco-manufactured equipment including routers, bridges, access and communication servers, and switches. Using CDP, you can view information about all the Cisco devices that are directly attached to the switch. In addition, CDP detects the native VLAN and port duplex mismatches.

The network management applications can retrieve the device type and SNMP-agent address of the neighboring Cisco devices using CDP. This feature enables the applications to send the SNMP queries to the neighboring devices. CDP allows the network management applications to discover the Cisco devices that are the neighbors of the already known devices, in particular, the neighbors running the lower-layer, transparent protocols.

CDP runs on all media that support Subnetwork Access Protocol (SNAP). CDP runs over the data link layer only.

The Cisco devices never forward the CDP packets. When new CDP information is received, the Cisco devices discard old information.

Default CDP Configuration

Table 31-1 shows the default CDP configuration.

Table 31-1 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


Note The CDP message interval can be configured in the range of 5-254 seconds.

Configuring CDP on the Switch

These sections describe how to configure CDP:

•Setting the CDP Global Enable and Disable States

•Setting the CDP Enable and Disable States on a Port

•Setting the CDP Message Interval

•Setting the CDP Holdtime

•Displaying CDP Neighbor Information

Setting the CDP Global Enable and Disable States

To set the CDP global enable state, perform this task in privileged mode:

 
Task
Command

Step 1 

Set the CDP global enable state on the switch.

set cdp {enable | disable}

Step 2 

Verify the CDP configuration.

show cdp

This example shows how to enable CDP globally and verify the configuration: 

Console> (enable) set cdp enable

CDP enabled globally

Console> (enable) show cdp

CDP               : enabled

Message Interval  : 60

Hold Time         : 180

Console> (enable)

This example shows how to disable CDP globally and verify the configuration: 

Console> (enable) set cdp disable 

CDP disabled globally

Console> (enable) show cdp

CDP               : disabled

Message Interval  : 60

Hold Time         : 180

Console> (enable)

Setting the CDP Enable and Disable States on a Port

You can enable or disable CDP on a per-port basis. You must enable CDP globally before the switch will transmit the CDP messages on any ports.

To set the CDP enable state on a per-port basis, perform this task in privileged mode:

 
Task
Command

Step 1 

Set the CDP enable state on the individual ports.

set cdp {enable | disable} [mod/port]

Step 2 

Verify the CDP configuration.

show cdp port [mod[/port]]

This example shows how to enable CDP on ports 3/1-2 and verify the configuration: 

Console> (enable) set cdp enable 3/1-2

CDP enabled on ports 3/1-2.

Console> (enable) show cdp port 3

CDP               : enabled

Message Interval  : 60

Hold Time         : 180


Port      CDP Status

--------  ----------

 3/1      enabled     

 3/2      enabled     

 3/3      disabled    

 3/4      disabled    

 3/5      disabled    

 3/6      disabled    

 3/7      enabled     

 3/8      enabled     

 3/9      enabled     

 3/10     enabled     

 3/11     enabled     

 3/12     enabled     

Console> (enable)

This example shows how to disable CDP on ports 3/1-6 and verify the configuration: 

Console> (enable) set cdp disable 3/1-6

CDP disabled on ports 3/1-6.

Console> (enable) show cdp port 3

CDP               : enabled

Message Interval  : 60

Hold Time         : 180


Port      CDP Status

--------  ----------

 3/1      disabled    

 3/2      disabled    

 3/3      disabled    

 3/4      disabled    

 3/5      disabled    

 3/6      disabled    

 3/7      enabled     

 3/8      enabled     

 3/9      enabled     

 3/10     enabled     

 3/11     enabled     

 3/12     enabled     

Console> (enable)

Setting the CDP Message Interval

The CDP message interval specifies how often the switch will transmit the CDP messages to the directly connected Cisco devices.

To set the default CDP message interval, perform this task in privileged mode:

 
Task
Command

Step 1 

Set the default CDP message interval. The allowed range is 5-900 seconds.

set cdp interval interval

Step 2 

Verify the CDP configuration.

show cdp

This example shows how to set the default CDP message interval to 100 seconds and verify the configuration: 

Console> (enable) set cdp interval 100    

CDP message interval set to 100 seconds for all ports.

Console> (enable) show cdp

CDP               : enabled

Message Interval  : 100

Hold Time         : 180

Console> (enable)

Setting the CDP Holdtime

The CDP holdtime specifies how much time can pass between the CDP messages from the neighboring devices before the device is no longer considered connected and the neighboring entry is aged out.

To set the default CDP holdtime, perform this task in privileged mode:

 
Task
Command

Step 1 

Set the default CDP holdtime. The allowed range is 10-255 seconds.

set cdp holdtime interval

Step 2 

Verify the CDP configuration.

show cdp

This example shows how to set the default CDP holdtime to 225 seconds and verify the configuration: 

Console> (enable) set cdp holdtime 225

CDP holdtime set to 225 seconds.

Console> (enable) show cdp

CDP               : enabled

Message Interval  : 100

Hold Time         : 225

Console> (enable)

Displaying CDP Neighbor Information

To display information about the directly connected Cisco devices, enter the show cdp neighbors command. Enter the vlan keyword to display the native VLAN for the connected ports. Enter the duplex keyword to display the duplex mode for the connected ports. Enter the capabilities keyword to display the device capability codes for the connected device. Enter the detail keyword to display the detailed information about the neighboring device.

Note If you enter the show cdp neighbors command for a device that supports earlier versions of CDP, "unknown" is displayed in the following fields: VTP Management Domain, Native VLAN, and Duplex.

To display information about the directly connected Cisco devices, perform this task:

Task
Command

Display information about the CDP neighbors.

show cdp neighbors [mod[/port]] [vlan | duplex | capabilities | detail]


This example shows how to display the CDP neighbor information for the connected Cisco devices: 

Console> (enable) show cdp neighbors

* - indicates vlan mismatch.

# - indicates duplex mismatch.

Port     Device-ID                       Port-ID                   Platform

-------- ------------------------------- ------------------------- ------------

 2/3     JAB023807H1(2948)               2/2                       WS-C2948

 3/1     JAB023806JR(4003)               2/1                       WS-C4003

 3/2     JAB023806JR(4003)               2/2                       WS-C4003

 3/5     JAB023806JR(4003)               2/5                       WS-C4003

 3/6     JAB023806JR(4003)               2/6                       WS-C4003

Console> (enable)

This example shows how to display the native VLAN for each port that is connected on the neighboring device (there is a native VLAN mismatch between port 3/6 on the local switch and port 2/6 on the neighboring device, as indicated by the asterisk [*]): 

Console> (enable) show cdp neighbors vlan

* - indicates vlan mismatch.

# - indicates duplex mismatch.

Port     Device-ID                       Port-ID                   NativeVLAN

-------- ------------------------------- ------------------------- ----------

 2/3     JAB023807H1(2948)               2/2                       522   

 3/1     JAB023806JR(4003)               2/1                       100   

 3/2     JAB023806JR(4003)               2/2                       100   

 3/5     JAB023806JR(4003)               2/5                       1     

 3/6     JAB023806JR(4003)               2/6*                      1     

Console> (enable)

This example shows how to display detailed information about the neighboring device: 

Console> (enable) show cdp neighbors 2/3 detail

Port (Our Port): 2/3     

Device-ID: JAB023807H1(2948)

Device Addresses:

  IP Address: 172.20.52.36

Holdtime: 132 sec

Capabilities: TRANSPARENT_BRIDGE SWITCH 

Version: 

  WS-C2948 Software, Version McpSW: 5.1(57) NmpSW: 5.1(1)

  Copyright (c) 1995-1999 by Cisco Systems, Inc.

Platform: WS-C2948

Port-ID (Port on Neighbors's Device): 2/2

VTP Management Domain: Lab_Network

Native VLAN: 522

Duplex: full

Console> (enable)