Now is as good a time as any to discuss Cisco Discovery Protocol.
CDP is a hot exam topic because it provides a means to discover information about network devices before any configuration has been applied. This is a very useful troubleshooting tool; however, it also presents a security risk.
CDP is Cisco proprietary, which means it will only work on Cisco devices. It is a Layer 2 service used by devices to advertise and discover basic information about directly connected neighbours. The IEEE version of CDP is Link Layer Discovery Protocol (LLDP), but this is not included in the CCNA syllabus.
Because CDP is a Layer 2 service it does not require IP addresses to be conifigured in order to exchange information. The interface need only be enabled. If an IP address is configured, then this will be included in the CDP message.
CDP is a very powerful troubleshooting tool and you will be expected to understand how to use it in the exam. Figure 4.1 below shows CDP outputs from Router 0. Imagine if you were asked to troubleshoot this network but had no topology diagram to work from.
Figure 4.1 – CDP Outputs from Router 0
The following configuration outputs correspond to Figure 4.1:
Router0#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 Interface Holdtime Capability Platform Port
Switch Fas 0/0 165 S 2960 Fas 0/1
Router Fas 0/1 169 R C1841 Fas 0/0
Router0#
You can see more information by adding the detail command to the end:
Router0#show cdp neighbors detail
Device ID: Switch
Entry address(es):
Platform: cisco 2960, Capabilities: Switch
Interface: FastEthernet0/0, Port ID (outgoing port): FastEthernet0/1
Holdtime: 178
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: Router
Entry address(es):
IP address : 192.168.1.2
Platform: cisco C1841, Capabilities: Router
Interface: FastEthernet0/1, Port ID (outgoing port): FastEthernet0/0
Holdtime: 122
Version :
Cisco IOS Software, 1841 Software (C1841-ADVIPSERVICESK9-M), Version 12.4(15)T1, RELEASE SOFTWARE (fc2)
Technical Support: http://www.cisco.com/techsupport
Copyright (c) 1986-2007 by Cisco Systems, Inc.
Compiled Wed 18-Jul-07 04:52 by pt_team
advertisement version: 2
Duplex: full
Now you can see the IOS release, model, IP address, and other information. Remember that you still haven’t configured an IP address on Router 0 yet.
We’ve already covered how to disable CDP on the device or interface only. Two other commands are show cdp, which displays protocol information for the device, and show cdp entry <Router>, which shows information about a specific device by imputting the name. I recommend that you spend some time checking CDP outputs during the labs you will configure in this guide.
Router0#show cdp
Global CDP information:
Sending CDP packets every 60 seconds
Sending a holdtime value of 180 seconds
Sending CDPv2 advertisements is enabled
Router0#show cdp ?
entry Information for specific neighbor entry
interface CDP interface status and configuration
neighbors CDP neighbor entries
traffic CDP statistics
| Output modifiers
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