It is used to show the Cisco Devices that have been connected to Cisco Switch.
Other vendor Devices will not be displayed in the table when you issue show cdp neighbors command to the switch.
Kopsiro(config)#cdp run
Kopsiro(config)#do 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
Switch Fas 0/2 170 S 2960 Fas 0/2
Router Gig 0/1 144 R C1900 Gig 0/0
IP Phone Fas 0/4 167 H P 7960
Switch Fas 0/5 159 3560 Fas 0/1
The above output gives so much about the devices connected to the switch, Local port on the switch, Model of the device port on the devices and Holdtime.
Take a look at the Toplogy above and the output we have. We are only getting 4 devices discovered by the cdp on the Switch yet from the Topology we have a total of 6 devices connected to the Switch.
What happened? Where is the desktop and the Cisco Firewall?
Ok, the reason why we are not able to get the Desktop computer is because CDP is a Cisco Proprietary protocol. A Desktop is not a Cisco device.
For the firewall, I am sure you wont like someone knowing about your firewall details on the network.
To avoid revealing details of your security device like a Firewall, Cisco didnt allow CDP learning from a Firewall.
Other Usefull Commands here are:
show cdp entry.
from the Show cdp neighbors above, the Device ID makes the entry.
If you want to check all the switches connected then you have the command as follows;
show cdp entry Switch as shown on the Output below
Kopsiro#show cdp entry Switch
Device ID: Switch
Entry address(es):
Platform: cisco 2960, Capabilities: Switch
Interface: FastEthernet0/2, Port ID (outgoing port): FastEthernet0/2
Holdtime: 175
Version :
Cisco IOS Software, C2960 Software (C2960-LANBASE-M), Version 12.2(25)FX, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2005 by Cisco Systems, Inc.
Compiled Wed 12-Oct-05 22:05 by pt_team
advertisement version: 2
Duplex: full
---------------------------
Device ID: Switch
Entry address(es):
Platform: cisco 3560, Capabilities: Switch
Interface: FastEthernet0/5, Port ID (outgoing port): FastEthernet0/1
Holdtime: 164
Version :
Cisco IOS Software, C3560 Software (C3560-ADVIPSERVICESK9-M), Version 12.2(37)SE1, RELEASE SOFTWARE (fc1)
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Thu 05-Jul-07 22:22 by pt_team
advertisement version: 2
Duplex: full
We can get so much of specific device from the above command. This is handy when you don't understand the Topology of your network.
The downside of it is that in case of a security breach, hackers can easily get the topology and target your devices with precision.
