Cisco Systems CCNA 2 manual Troubleshooting Router Issues Overview, Troubleshooting using show cdp

9.3 Troubleshooting Router Issues Overview

Essential Labs:	9.3.4, 9.3.5, and 9.3.7
Optional Labs:	None
Core TIs:	All
Optional TIs:	none

Course-Level Claim: Students can use the commands incorporated within IOS to analyze and rectify network problems.

Certification-Level Claim: Students can perform simple WAN troubleshooting.

Hands-on skills: none

9.3.1 Troubleshooting Layer 1 using show interfaces

The Cisco IOS contains many troubleshooting commands. The show commands are widely used. The show interfaces command is used to check the status and statistics of the interfaces. The show interfaces serial command displays the line and data link protocol status. It also provides information to help diagnose other Layer 1 issues that are not as easy to determine. These problems include line interruptions, faulty switch, faulty DSU, or faulty router hardware. Have the students view the results displayed by these commands and discuss the results to make sure they understand the output. This is an important concept and valuable tool for troubleshooting.

9.3.2 Troubleshooting Layer 2 using the show interfaces

The show interfaces command is possibly the most important tool to discover Layer 1 and Layer 2 problems in the router. The first parameter refers to the physical layer. The second parameter indicates if the IOS processes that control the line protocol consider the interface to be usable. This is determined by whether keepalives are successfully received. If the interface misses three consecutive keepalives, the line protocol is marked as down. The show interfaces serial command should be used after a serial interface is configured to verify the changes and that the interface is operational.

View these results with the students and discuss the results to make sure they understand the output. This is an important concept and valuable tool for troubleshooting. The students are not expected to understand all the fields contained in the show interfaces command output. This command will be revisited in CCNA 4 and more information will be presented.

9.3.3 Troubleshooting using show cdp

CDP advertises device information to its direct neighbors that includes MAC and IP addresses and outgoing interfaces. The output from the show cdp neighbors command displays information about directly connected neighbors. This information can be used to debug connectivity issues. The show cdp neighbor detail command returns specific device details such as the active interfaces, the port ID, and the device. The version of Cisco IOS that is used on the remote devices is also shown.

107 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 9 Copyright ♥ 2004, Cisco Systems, Inc.