Cisco Discovery Protocol (CDP)

CDP is a Cisco proprietary data link layer protocol that operates over any medium that supports the Subnetwork Access Protocol (SNAP) encapsulation (LANs, most WANs, and ATM). It is important to understand that because CDP operates at Layer 2 (data link layer of the OSI model), it functions independently of the Layer 3 (network) protocol (IP or IPX). CDP is on by default, but it can be disabled. In many cases, CDP is disabled on dial backup links, such as ISDN, so as to not keep the link up constantly. Chapter 4 of Interconnecting Cisco Network Devices provides more detailed information about CDP.
To display what CDP has discovered, issue this command:

Router#show cdp [entry | interface | neighbors | traffic]

This command offers several options. For purposes here, look at the interface and neighbors options only, but feel free to become familiar with the other options.
The first option to look at is the interface option:

Router#show cdp interface [ethernet | loopback | null | serial] [number]

The first option on this command is to specify the type of interface that you want to see CDP information on. The last option is to specify the interface number. Example 7-44 uses the show cdp interface command to examine R5's Serial 0 interface.

Example 7-44. show cdp interface serial 0 Command Output

R5#show cdp interface serial 0
Serial0 is up, line protocol is up
  Encapsulation HDLC
  Sending CDP packets every 60 seconds
  Holdtime is 180 seconds
R5#

It does not give a whole lot of information, but it is a "quick and dirty" way of seeing some CDP information. The second option of the show cdp command that we are going to look at is this:

Router#show cdp neighbors [bri | loopback | null | serial | tokenring | detail]

As you can see, this command provides the option to see CDP information by interface, but the last option, detail, gives a complete summary of all Cisco devices that CDP was capable of discovering and displays information about those devices. Example 7-45 displays the output of the show cdp neighbors detail command.

Example 7-45. show cdp neighbors detail Command Output

R5#show cdp neighbors detail
-------------------------
Device ID: R3
Entry address(es):
  IP address: 192.168.35.3
Platform: cisco 2500, Capabilities: Router
Interface: Serial0, Port ID (outgoing port): Serial1
Holdtime : 164 sec

Version :
Cisco Internetwork Operating System Software
IOS (tm) 2500 Software (C2500-JS-L), Version 11.2(17), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1999 by cisco Systems, Inc.
Compiled Mon 04-Jan-99 17:27 by ashah

R5#

The first piece of information that the output shows is the device ID, usually the hostname of the device. As you can see, R5 has discovered R3.
The second field is the IP address of R3. This is very useful if you have several routers and you are not sure of the IP address of the desired router. You can use this command to find that instead of trying to track down a network map.
The third field is the platform and capability. R3 is a 2500 and is a router. If you issue this same command on R1, you will see the Catalyst 1900 switch in the summary as well.
The fourth item is the interface on R5 that the device was discovered and the port on R3 to which R5 is connected. The command also displays the Cisco IOS Software version of your neighbor.
You will see this command revisited throughout the rest of the book, to demonstrate the different ways to utilize this command to help configure and troubleshoot the network.
Now that you have all the interfaces configured and have established IP connectivity, you can move on to the next chapter and start configuring the different routing protocols.

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