Cisco Discovery Protocol - CDP

This weekend was yet another CCNA Laboratory session, this time around it was divided into two:

1. Using CDP to gather information
2. Building a Hosts table in a router

A Cisco Discovery Protocol (CDP) is a protocol which was designed by Cisco guys to promote “spying” on neighbouring devices by network administrators. Well, of course, not the Alex Litvinenko spying but spying as in being used to collect information about attached and remote devices, this could be hardware information, protocol information etc.

Cisco uses the commands such as:

• “show cdp neighbors” : to display information about other devices connected to your device. Using the configuration in Basic Router and Switch Configuration Lab, suppose that you are in Router 2 (PxR2) and you want to check all the devices connected to that router: The much simplier way will be to use the command

PxR2#show cdp neighbors
which outputs the information :
Capability Codes: R - Router, T - Trans Bridge, B - Source Route Bridge
S - Switch, H - Host, I - IGMP, r - Repeater

Device ID	Local Intrfce	Holdtime	Capability	Platform	Port ID
PxR1	Ser 0/0/0	167	R	2610	Ser 0/0
PxS1	Fas 0/0	134	S I	WS-C2950-	Fas 0/1

A switch, PxS1 and one router, PxR1 are the only devices connected to router 2, PxR2.


• Obviously the information is not of much value, so that’s why Cisco came up with another command “show cdp neighbor detail” which provides a much more detailed information about connected devices such as their IP Addresses, Cisco version, holdtime etc are revealed.
  

PxR2#show cdp neighbor detail
————————-
Device ID: PxR1
Entry address(es):
IP address: 172.16.17.1
Platform: cisco 2610, Capabilities: Router
Interface: Serial0/0/0, Port ID (outgoing port): Serial0/0
Holdtime : 124 sec

Version :
Cisco Internetwork Operating System Software
IOS ™ C2600 Software (C2600-JK9O3S-M), Version 12.2(24), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2004 by cisco Systems, Inc.
Compiled Wed 28-Apr-04 15:30 by kellmill

advertisement version: 2

————————-
Device ID: PxS1
Entry address(es):
IP address: 192.168.27.1
Platform: cisco WS-C2950-24, Capabilities: Switch IGMP
Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/1
Holdtime : 151 sec

Version :
Cisco Internetwork Operating System Software
IOS ™ C2950 Software (C2950-I6Q4L2-M), Version 12.1(11)EA1, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2002 by cisco Systems, Inc.
Compiled Wed 28-Aug-02 10:25 by antonino

advertisement version: 2
Protocol Hello: OUI=0×00000C, Protocol ID=0×0112; payload len=27, value=00000000FFFFFFFF010221FF000000000000000
9E40FF0000
VTP Management Domain: ‘null’
Duplex: full

• The “show cdp ?” was rather useful when I needed to get any Cisco Discovery Protocol command. For instance

PxR2#show cdp ?
entry - Information for specific neighbor entry
interface - CDP interface status and configuration
neighbors - CDP neighbor entries
traffic - CDP statistics
| - Output modifiers

The next article will be about the second part of this laboratory session, that is, Building a Hosts table. In addition to that, I will talk about how I collected information of neighbouring devices (using Cisco Discovery Protocol + Hosts table) as well as obtaining information of neighbouring devices of neighbours and neighbouring devices of neighbour’s neighbouring devices; We could be here all day, I guess you see where I’m going with this.

Thanks for your comments