1. Learning Objectives

This configuration exercise covers:

1. Understanding CDP protocol characteristics
2. CDP configuration and troubleshooting techniques
3. Viewing directly connected neighbor information via CDP

2. Network Topology

The laboratory topology for CDP configuration is illustrated below.

3. Configuration Steps

(1) Configure RouterA

RouterA(config)#interface serial 0/1/0
RouterA(config-if)#ip address 192.168.10.1 255.255.255.0
RouterA(config-if)#no shutdown
RouterA(config-if)#exit
RouterA(config)#interface fastEthernet 0/0
RouterA(config-if)#ip address 192.168.20.1 255.255.255.0
RouterA(config-if)#no shutdown 
RouterA(config-if)#exit
RouterA(config)#

(2) Configure RouterB

RouterB(config)#interface serial 0/1/0
RouterB(config-if)#ip address 192.168.10.2 255.255.255.0
RouterB(config-if)#no shutdown 
RouterB(config-if)#exit

(3) Configure SwitchA

SwitchA(config)#interface vlan 1
SwitchA(config-if)#ip address 192.168.20.100 255.255.255.0
SwitchA(config-if)#no shutdown 
SwitchA(config-if)#exit

4. Verification and Troubleshooting

(1) Display CDP Operational Status

RouterA#show cdp                      
Global CDP information:
    Sending CDP packets every 60 seconds
    Sending a holdtime value of 180 seconds
    Sending CDPv2 advertisements is enabled
RouterA#

The output confirms that CDP is enabled by default, transmitting CDP messages every 60 seconds from all eligible interfaces. In Ethernet networks, these messages use IEEE 802.3 encapsulation with a destination MAC address of 0100.0ccc.ccee (multicast address). When a neighbor receives CDP messages, it stores this inforamtion in its local CDP table with a 180-second holdtime. The current implementation uses CDP version 2.

(2) Display Interface CDP Status

RouterA#show cdp interface fastEthernet 0/0        
FastEthernet0/0 is up, line protocol is up
  Sending CDP packets every 60 seconds           
  Holdtime is 180 seconds                        
RouterA#

(3) Display Connected Neighbor Information

RouterA#show cdp neighbors               
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
                  S - Switch, H - Host, I - IGMP, r - Repeater, P - Phone
Device ID    Local Intrfce   Holdtme    Capability   Platform    Port ID
RouterB      Ser 0/1/0        155            R       C2900       Ser 0/1/0
SwitchA      Fas 0/0          145                    3650        Fas 1/0/1
RouterA#

This output indicates that RouterA has two CDP neighbors: RouterB and SwitchA. The field definitions are as follows:

1. Device ID: Hostname of the CDP neighbor device
2. Local Interface: The local interface through which RouterA connects to the CDP neighbor
3. Holdtme: Remaining validity period since the last CDP message was received from the neighbor (counts down from 180 seconds)
4. Capability: Device type and capabilities of the neighbor; the Capability Codes header explains each code
5. Platform: Hardware model of the neighbor device
6. Port ID: The interface on the neighbor device to which RouterA is connetced

(4) Display Detailed Neighbor Information

RouterB#show cdp neighbors detail       

Device ID: RouterA                      
Entry address(es):                 
  IP address : 192.168.10.1
Platform: cisco C2900, Capabilities: Router
Interface: Serial0/1/0, Port ID (outgoing port): Serial0/1/0
Holdtime: 169

Version :
Cisco IOS Software, C2900 Software (C2900-UNIVERSALK9-M), Version 15.1(4)M4, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2012 by Cisco Systems, Inc.
Compiled Thurs 5-Jan-12 15:41 by pt_team

advertisement version: 2       
Duplex: full

RouterB#

The show cdp neighbors detail command provides identical functionality to show cdp entry *, where the asterisk can be replaced with a specific device name. The clear cdp table command removes all CDP cache entries.

(5) Disable CDP and Adjust CDP Parameters

RouterA(config)#interface fastEthernet 0/0
RouterA(config-if)#no cdp enable 
RouterA(config-if)#exit
RouterA(config)#no cdp run
RouterA(config)#cdp run
RouterA(config)#cdp timer 10   
RouterA(config)#cdp holdtime 30
RouterA(config)#exit

(6) Display CDP Traffic Statistics

RouterA#show cdp traffic