Module 4: Overview / Learning about Other Devices / Introduction to CDP / Information obtained with CDP

Overview

Sometimes network documentation is incomplete or inaccurate. Cisco Discovery Protocol (CDP) is a useful tool in these situations because it can build a basic picture of a network. CDP is a media and protocol independent, Cisco proprietary protocol used for neighbor discovery. CDP will only show information about directly connected neighbors but it is still a powerful tool.

After a router is initially configured it is often difficult to connect directly to the router for configuration changes or other activities. Telnet is a TCP/IP-based application that allows remote connection to the router command-line interface (CLI) for configuration, monitoring, and troubleshooting purposes. Telnet is an essential tool for network professionals.

This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams. -

Students who complete this module should be able to perform the following tasks:

• Enable and disable CDP
• Use the show cdp neighbors command
• Determine which neighbor devices are connected to each local interface
• Gather network address information about neighbor devices that use CDP
• Establish a Telnet connection
• Verify a Telnet connection
• Disconnect from a Telnet session
• Suspend a Telnet session
• Perform alternative connectivity tests
• Troubleshoot remote terminal connections 

 
4.1 Discovering and Connecting to Neighbors

Introduction to CDP 

4.1.1 This page will introduce Cisco Discovery Protocol (CDP). CDP is a Layer 2 protocol that connects lower physical media and upper network layer protocols, as shown in Figure  . CDP is used to obtain information about neighboring Cisco devices, such as the types of devices connected, the router interfaces they are connected to, the interfaces used to make the connections, and the model numbers of the devices. CDP is media and protocol independent, and runs on all Cisco equipment over the Subnetwork Access Protocol (SNAP).

CDP Version 2 (CDPv2) is the most recent release of the protocol. Cisco IOS Release 12.0(3)T or later supports CDPv2. CDP Version 1 (CDPv1) is enabled by default with Cisco IOS Release 10.3 to 12.0(3)T.

When a Cisco device boots up, CDP starts up automatically and allows the device to detect neighbor devices that use CDP. CDP operates at the data link layer and allows two systems to learn about each other, even if they use different network layer protocols.

Each device that is configured for CDP sends periodic messages, which are known as advertisements, to directly connected Cisco devices. Each device advertises at least one address at which it can receive Simple Network Management Protocol (SNMP) messages. The advertisements also contain time-to-live or holdtime information, which indicates the length of time that receiving devices should hold CDP information before they discard it. Each device also listens to periodic CDP messages that are sent by others to learn about neighbor devices.

The next page will explain how CDP collects and delivers information. 
Information obtained with CDP 

4.1.2 This page will explain how CDP is used to obtain information about network devices.

The primary use of CDP is to discover all Cisco devices that are directly connected to a local device. Use the show cdp neighbors command to display CDP updates on the local device.

Figure displays an example of how CDP delivers its collection of information to a network administrator. Each router that uses CDP exchanges protocol information with its neighbors. The network administrator can display the results of this CDP information exchange on a console that is connected to a local router.

An administrator can use the show cdp neighbors command to display information about the networks that are directly connected to a router. CDP transmits type length values (TLVs) to provide information about each CDP neighbor device. TLVs are blocks of information embedded in CDP advertisements.

Device TLVs displayed by the show cdp neighbors command include the following:

• Device ID
• Local Interface
• Holdtime
• Capability
• Platform
• Port ID

The following TLVs are only included in CDPv2:

• VTP management domain name
• Native VLAN
• Full or half-duplex

Notice that the router at the bottom of Figure is not directly connected to the console router that is used by the administrator. To obtain CDP information about this device, the administrator would need to Telnet to a router that is directly connected to this device.

The next page will introduce the commands that are used to monitor CDP information.