Catalyst 4500 Series Software Configuration Guide, 8.3 GLX and 8.4GLX
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 State

Setting the CDP Enable State 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 enterprise LAN switches.

Note For complete syntax and usage information for the commands that are used in this chapter, refer to the Catalyst 4500 Series, Catalyst 2948G, Catalyst 2948G-GE-TX, and Catalyst 2980G Switches Command Reference.

This chapter consists of these sections:

•Understanding How CDP Works

•Default CDP Configuration

•Configuring CDP on the Switch

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 that are 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 feature allows 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). CDP runs over the data link layer only.

Cisco devices do not forward CDP packets. When new CDP information is received, old information is discarded.

CDP was enhanced in software release 8.3(1)GLX to facilitate backwards 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 additional information to ensure that it does not oversubscribe the available power.

Default CDP Configuration

Table 21-1 shows the default CDP configuration.

Table 21-1 CDP Default Configuration 

Feature
Default Value

CDP global enable state

Enabled

CDP port enable state

Enabled on all ports

CDP message interval

60 sec

CDP holdtime

180 sec


Configuring CDP on the Switch

The following sections describe how to configure CDP.

Setting the CDP Global Enable State

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

 
Task
Command

Step 1 

Set the CDP global enable state.

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 State on a Port

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

set cdp {enable | disable} [mod_num/port_num]

Step 2 

Verify the CDP configuration.

show cdp port [mod_num[/port_num]]

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)

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)

Setting the CDP Message Interval

The CDP message interval specifies how often the switch will transmit CDP messages to 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 CDP messages from neighboring devices before the device is no longer considered connected and the neighbor 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 directly connected Cisco devices, enter the show cdp neighbors command.

To display specific information, use the following keywords:

•To display the native VLAN for the connected ports, enter the vlan keyword.

•To display the duplex mode for the connected ports, enter the duplex keyword.

•To display the device capability codes for the connected device, enter the capabilities keyword.

•To display the device capability codes for the connected device, enter the detail keyword.

To display information about directly connected Cisco devices, perform this task in privileged mode:

Task
Command

View information about CDP neighbors.

show cdp neighbors [mod_num[/port_num]] [vlan | duplex | capabilities | detail]


This example shows how to display CDP neighbor information for 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 neighbor 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)