Configuring CDP

This chapter describes how to configure Cisco Discovery Protocol (CDP) on the Catalyst 3750 switch. Unless otherwise noted, the term switch refers to a standalone switch and to a switch stack.

Note For complete syntax and usage information for the commands used in this chapter, see the command reference for this release and the "System Management Commands" section in the Cisco IOS Configuration Fundamentals Command Reference, Release 12.2.

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 switch, CDP enables Network Assistant 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.

CDP and Switch Stacks

A switch stack appears as a single switch in the network. Therefore, CDP discovers the switch stack, not the individual stack members. The switch stack sends CDP messages to neighboring network devices when there are changes to the switch stack membership, such as stack members being added or removed.

Configuring CDP

These sections contain this configuration information:

•Default CDP Configuration

•Configuring the CDP Characteristics

•Disabling and Enabling CDP

•Disabling and Enabling CDP on an Interface

Default CDP Configuration

Table 27-1 shows the default CDP configuration.

Table 27-1 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 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 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 your settings.

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 CDP characteristics. 

Switch# configure terminal

Switch(config)# cdp timer 50

Switch(config)# cdp holdtime 120

Switch(config)# cdp advertise-v2

Switch(config)# end

 
   
 
    
   
 For additional CDP show commands, see the "Monitoring and Maintaining CDP" section. 
 
     
    
    
   
 Disabling and Enabling CDP 
 
    
   
 CDP is enabled by default. 
 
   
 
     
   
 
   
 
    
   
 Note Switch clusters and other Cisco devices (such as Cisco IP Phones) regularly exchange CDP messages. Disabling CDP can interrupt cluster discovery and device connectivity. For more information, see Chapter 7 "Clustering Switches" and see Getting Started with Cisco Network Assistant, available on Cisco.com. 
 
   
 
    
    
    
   
 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 to receive CDP information. 
 
    
    
   
 Beginning in privileged EXEC mode, follow these steps to disable CDP on a port: 
 
    
    
   
 
 
   
 
    
 
      
      
      
 
         
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
 Step 1  
 		 
       
 configure terminal 
 		 
       
 Enter global configuration mode. 
 		 
      
 
      
 
       
 Step 2  
 		 
       
 interface interface-id 
 		 
       
 Specify the interface on which you are disabling CDP, and enter interface configuration mode. 
 		 
      
 
      
 
       
 Step 3  
 		 
       
 no cdp enable  
 		 
       
 Disable CDP on the 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 a port when it has been disabled: 
 
    
   
 
 
   
 
    
 
      
      
      
 
         
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
 Step 1  
 		 
       
 configure terminal 
 		 
       
 Enter global configuration mode. 
 		 
      
 
      
 
       
 Step 2  
 		 
       
 interface interface-id 
 		 
       
 Specify the interface on which you are enabling CDP, and enter interface configuration mode. 
 		 
      
 
      
 
       
 Step 3  
 		 
       
 cdp enable  
 		 
       
 Enable CDP on the 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 a port when it has been disabled. 
 
    
   
 
    
Switch# configure terminal

 
   
 
    
   
 
    
Switch(config)# interface gigabitethernet1/0/1

 
   
 
    
   
 
    
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 [interface-id] 
 		 
       
 Display information about interfaces where CDP is enabled. 
 
 You can limit the display to the interface about which you want information. 
 		 
      
 
      
 
       
 show cdp neighbors [interface-id] [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 of a specific 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.