Configuring CDP

This chapter contains information about how to configure Cisco Discovery Protocol (CDP) on the Catalyst 6500 series switches, which supplements the information in these publications:

•The Cisco IOS Configuration Fundamentals Configuration Guide, Release 12.2, "System Management," "Configuring Cisco Discovery Protocol (CDP)" at this URL:

http://www.cisco.com/en/US/docs/ios/12_2/configfun/configuration/guide/fcf015.html

•The Cisco IOS Configuration Fundamentals Command Reference, Release 12.2, "System Management Commands," "CDP Commands" publication at this URL:

http://www.cisco.com/en/US/docs/ios/12_2/configfun/command/reference/frf015.html

This chapter consists of these sections:

•Understanding How CDP Works

•Configuring CDP

Understanding How CDP Works

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

CDP runs on all LAN and WAN media that support Subnetwork Access Protocol (SNAP).

Each CDP-configured device sends periodic messages to a multicast address. Each device advertises at least one address at which it can receive SNMP messages. The advertisements also contain the time-to-live, or holdtime information, which indicates the length of time a receiving device should hold CDP information before discarding it.

Configuring CDP

These sections describe how to configure CDP:

•Enabling CDP Globally

•Displaying the CDP Global Configuration

•Enabling CDP on a Port

•Displaying the CDP Interface Configuration

•Monitoring and Maintaining CDP

Enabling CDP Globally

To enable CDP globally, perform this task:

Command
Purpose

Router(config)# cdp run

Enables CDP globally.

Router(config)# no cdp run

Disables CDP globally.


This example shows how to enable CDP globally: 

Router(config)# cdp run

Displaying the CDP Global Configuration

To display the CDP configuration, perform this task:

Command
Purpose

Router# show cdp

Displays global CDP information.


This example shows how to display the CDP configuration: 

Router# show cdp

Global CDP information:

        Sending CDP packets every 120 seconds

        Sending a holdtime value of 180 seconds

        Sending CDPv2 advertisements is enabled

Router# 

 
   
 
    
   
 For additional CDP show commands, see the "Monitoring and Maintaining CDP" section. 
 
     
    
    
    
   
 Enabling CDP on a Port 
 
    
   
 To enable CDP on a port, perform this task: 
 
    
    
   
 
 
   
 
    
 
      
      
      
 
         
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
 Step 1  
 		 
       
Router(config)# interface {{type1  slot/port} | {port-channel number}} 
 
       
 Selects the port to configure. 
 		 
      
 
      
 
       
 Step 2  
 		 
       
Router(config-if)# cdp enable 
 
       
 Enables CDP on the port. 
 		 
      
 
      
 
       
Router(config-if)# no cdp enable 
 
       
 Disables CDP on the port. 
 		 
      
 
      
    
 
    
 
      
      
 
       
 1 type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet 
 		 
      
 
      
    
 
   
 
   
 
 
    
   
 This example shows how to enable CDP on Fast Ethernet port 5/1: 
 
    
   
 
    
Router(config)# interface fastethernet 5/1

 
   
 
    
   
 
    
Router(config-if)# cdp enable

 
   
 
     
    
    
   
 Displaying the CDP Interface Configuration 
 
    
   
 To display the CDP configuration for a port, perform this task: 
 
    
    
   
 
 
   
 
    
 
      
      
      
 
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
Router# show cdp interface [{{type1  slot/port} | {port-channel number}}] 
 
       
 Displays information about ports where CDP is enabled. 
 		 
      
 
      
    
 
    
 
      
      
 
       
 1 type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet 
 		 
      
 
      
    
 
   
 
   
 
   
 
 
    
   
 This example shows how to display the CDP configuration of Fast Ethernet port 5/1: 
 
    
   
 
    
Router# show cdp interface fastethernet 5/1

 
   
 
    
   
 
    
FastEthernet5/1 is up, line protocol is up

 
   
 
    
   
 
    
  Encapsulation ARPA

 
   
 
    
   
 
    
  Sending CDP packets every 120 seconds

 
   
 
    
   
 
    
  Holdtime is 180 seconds

 
   
 
    
   
 
    
Router# 

 
   
 
     
    
    
    
   
 Monitoring and Maintaining CDP 
 
    
   
 To monitor and maintain CDP on your device, perform one or more of these tasks: 
 
    
    
    
    
    
    
    
    
    
   
 
 
   
 
    
 
      
      
      
 
        
        

          Command 
        
 		 
        
        

          Purpose 
        
 		 
      
 
      
 
       
Router# clear cdp counters 
 
       
 Resets the traffic counters to zero. 
 		 
      
 
      
 
       
Router# clear cdp table 
 
       
 Clears information about neighbors from the CDP table. 
 		 
      
 
      
 
       
Router# show cdp 
 
       
 Displays global information such as frequency of transmissions and the holdtime for packets being transmitted. 
 		 
      
 
      
 
       
Router# show cdp entry entry_name [protocol | version] 
 
       
 Displays information about a specific neighbor. The display can be limited to protocol or version information. 
 		 
      
 
      
 
       
Router# show cdp interface [type1  slot/port}] 
 
       
 Displays information about interfaces on which CDP is enabled. 
 		 
      
 
      
 
       
Router# show cdp neighbors [type1 slot/port] [detail] 
 
       
 Displays information about neighbors. The display can be limited to neighbors on a specific interface and expanded to provide more detailed information. 
 		 
      
 
      
 
       
Router# show cdp traffic 
 
       
 Displays CDP counters, including the number of packets sent and received and checksum errors. 
 		 
      
 
      
 
       
Router# show debugging 
 
       
 Displays information about the types of debugging that are enabled. Refer to the Debug Command Reference for more information about CDP debug commands. 
 		 
      
 
      
    
 
    
 
      
      
 
       
 1 type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet 
 		 
      
 
      
    
 
   
 
   
 
   
 
 
    
   
 This example shows how to clear CDP counter configuration: 
 
    
   
 
    
Router# clear cdp counters

 
   
 
    
   
 
    
 
   
 
    
   
 This example shows how to display information about the neighboring equipment: 
 
    
   
 
    
Router# show 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

 
   
 
    
   
 
    
JAB023807H1      Fas 5/3            127         T S       WS-C2948  2/46

 
   
 
    
   
 
    
JAB023807H1      Fas 5/2            127         T S       WS-C2948  2/45

 
   
 
    
   
 
    
JAB023807H1      Fas 5/1            127         T S       WS-C2948  2/44

 
   
 
    
   
 
    
JAB023807H1      Gig 1/2            122         T S       WS-C2948  2/50

 
   
 
    
   
 
    
JAB023807H1      Gig 1/1            122         T S       WS-C2948  2/49

 
   
 
    
   
 
    
JAB03130104      Fas 5/8            167         T S       WS-C4003  2/47

 
   
 
    
   
 
    
JAB03130104      Fas 5/9            152         T S       WS-C4003  2/48