Lab 4: CDP

Objective:   To understand how the Cisco Discovery Protocol functions and what it takes

for Cisco devices to be discovered.

Lab Equipment:   We will be using Router 1 & Router 4.  To select Router 1 click on the

click on the button Router1 at the top of your screen. 

1. On Router 1, enter global configuration mode 

Router>enable

Router#conf t

Router(config)# 

2. On Router 1, and change the hostname to R1

Router(config)#hostname R1 

R1(config)# 

3. Select Router 4 by clicking on the button Router4 at the top of your screen, and change

the hostname to R4 

Router>enable 

Router#conf t 

Router(config)#hostname R4

R4(config)#

Note:  By default, all interfaces are shutdown (disabled). 

4. Enable the Serial 0 interface on R1.

R1(config)#interface Serial 0 

R1(config-if)#no shutdown 

5. Now enable the Serial 0 interface on R4

R4(config)#interface Serial 0 

R4(config-if)#no shutdown 

6. Enable the Ethernet 0 interface on R1.

R1(config)#interface Ethernet 0 

R1(config-if)#no shutdown

CDP allows devices to share basic configuration information.  CDP will operate without

any protocol specific information configured.  CDP is enabled by default on all

interfaces. CDP is a Data link Protocol occurring at Layer 2 of the OSI model.   This is

important to understand because CDP is not routable.  It can only traverse to directly

connected devices.

7. On R1, type the command to give the status of all interfaces that are running CDP.

R1(config-if)#exit 

R1(config)#exit 

R1#show cdp interface

The sample output below shows that both interfaces are up and sending CDP packets.

Serial0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 60 seconds

Holdtime is 180 seconds 

<output omitted> 

R1# 

Now that the router has interfaces that are broadcasting and receiving CDP updates we

can use CDP to find out about directly connected neighbors. 

8. On R1, type the command to provide information about directly connected neighbors.

R1#show cdp neighbors

Below is some sample output

Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge

S - Switch, H - Host, I - IGMP, r - Repeater

Device ID     Local Interface      Holdtme      Capability      Platform      Port ID

R4                      Serial 0                148                  R                1700         Serial 0

R1#

The first device on the list for R1 is R4 via the Serial0 link.  R1 is receiving CDP updates

from R4, the updates tell R1 to hold on to the information for a specified amount of time.

 At the time this command was pressed there were 148 seconds left in the hold time for

R1's update.  If that time expires before receiving another update R1's information will be

removed from the table.  R4 is a 1000 series router; this is shown in the platform column.

 The final column, Port ID, is the port on the other device from which the updates are

being sent. 

9. On R1, type the command to provide more detailed information about directly

connected neighbors.

R1#show cdp neighbors detail

Below is some sample output

Device ID: R4

Entry address(es):

Platform: cisco 2501, Capabilities: Router

Interface: Serial0, Port ID (outgoing port): Serial0

Holdtime : 162 sec

Version :

Cisco Internetwork Operating System Software

Software, Version 12.0(16), RELEASE SOFTWARE (fc2)

Copyright (c) 1986-2001 by cisco Systems, Inc.

Compiled Fri 02-Mar-01 17:34 by dchih

This command shows devices one at a time. It is used to display Network Layer address

information. At this point there are no configured IP, IPX or AppleTalk addresses so this

field is blank. The command also displays IOS version information. Notice that the

devices are listed in order. If one wants to find out information about a device further

down the list, one would need to scroll down using the space bar.

10. On R1, type the command to provide information about the specific device "R4"

R1#show cdp entry R4

The below is sample output

Device ID: R4

Entry address(es):

Platform: cisco 1000, Capabilities: Router

Interface: Serial0, Port ID (outgoing port): Serial0

Holdtime : 148 sec

Version :

Cisco Internetwork Operating System Software

Software, Version 12.0(16), RELEASE SOFTWARE (fc2)

Copyright (c) 1986-2001 by cisco Systems, Inc.

Compiled Fri 02-Mar-01 17:34 by dchih

R1# 

This command gives the same information as the show cdp neighbor detail command, but

allows a single device to be specified.  Also notice that this is one of the only case-

sensitive commands that exist.

11. On R1, type the command to see how often CDP updates are being sent and how long

a recipient is to hold on to the update.

R1#show cdp

The below is sample output

Global CDP information:

Sending CDP packets every 60 seconds

Sending a holdtime value of 180 seconds  

Sending CDPv2 advertisements is enabled

12. On R1, type the command to adjust the amount of seconds between CDP updates to

45.

R1#conf t 

R1(config)#cdp timer 45 

 Besides the update interval, the holdtime value may also be adjusted.  This value tells the

recipient of the update how long to hold on to the CDP information in the update.  It is

also a global parameter. 

13. On R1, type the command to adjust the holdtime timer to 60 seconds.

R1#conf t 

R1(config)#cdp holdtime 60

14. On R1, type the command to verify the changes made.

R1#show cdp

The below is sample output

R1#sh cdp

Global CDP information:

Sending CDP packets every 45 seconds

Sending a holdtime value of 60 seconds 

Sending CDPv2 advertisements is enabled

R1# 

If there are no other directly connected Cisco devices on the network, or to simply

conserve bandwidth, CDP can be disabled. 

15. On R1, type the command to disable CDP for the ENTIRE router.

R1#conf t 

R1(config)#no cdp run 

16. On R1, type the command to turn CDP back on for the ENTIRE router.

R1#conf t 

R1(config)#cdp run

At times you may wish to disable CDP for a specific interface, for example a very low-

bandwidth interface, or security reasons. 

17. On R1, Disable CDP for only the specific interface Ethernet 0.

R1(config)#interface Ethernet 0 

R1(config-if)#no cdp enable

18. On R1, type the command to verify that Ethernet 0 is no longer sending CDP updates

(You can verify that the interface Ethernet 0 is not sending cdp updates because it does

not show up as an entry in the output).

R1#show cdp interface

Below is sample output from the command.

R1#show cdp interface

Serial0 is up, line protocol is up

Encapsulation HDLC

Sending CDP packets every 45 seconds

Holdtime is 60 seconds  

Basic Lab Summary

This lab is designed to introduce the Cisco Discovery Protocol (CDP) and some of its

available commands.

The Cisco Discovery Protocol (CDP) Discovery Protocol

CDP allows devices to share basic configuration information without even configuring

any protocol specific information and is enabled by default on all interfaces.

CDP is a Datalink Protocol occurring at Layer 2 of the OSI model.   

This is important to understand because CDP is not routable and can only traverse to

directly connected devices.

CDP allows you to view information such Operating System Version, Protocol

Information, and much more.   

This can be very handy for troubleshooting a variety of problems.

CDP Configuration by default it is enabled on the router and all interfaces.