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Structured Network Troubleshooting Methodology Step 2 Establish a Theory of Probable Cause by Anthony Sequeira

If a troubleshooter does not follow a structured approach, the temptation is to move between troubleshooting tasks in a fairly random way, often based on instinct. Although such an approach might well lead to problem resolution, it can become confusing to remember what you have tried and what you have not tried. Also, if another administrator comes to assist you, communicating to that other administrator the steps you have already gone though could be challenging.

Therefore, following a structured troubleshooting approach not only helps prevent you from trying the same thing more than once and inadvertently skipping a task, but also aids in communicating to someone else the possibilities you have already eliminated.

Step 2 Establish a Theory of Probable Cause

This is the point in the troubleshooting process where your experience and intuition can be extremely helpful because it is when you brainstorm a list of possible causes. As you brainstorm, be sure to question the obvious. Also, think in terms of top-to-bottom troubleshooting (moving from top to bottom within the OSI model) or bottom-to-top troubleshooting. Alternatively, you could use a divide-and-conquer troubleshooting approach.

When examining your collected data (for example, output from ipconfig /all command), question everything. For example, you might think that the issue described in step 1 (identify the problem) could result from causes such as an ACL blocking traffic to or from the PC, a connectivity issue with the PC or server, or an incorrect IP address configuration on the PC.

From your list of possible causes, select the one you consider the most likely. From the previous list, you might believe that an incorrect IP address configuration on the PC is the most likely cause of the problem. Specifically, you might conclude that the issue is not related to connectivity because other PCs can get to the server, and user A's PC can get to other servers. Also, you might conclude that it is more likely that user A's PC has a bad IP address configuration than that an ACL has been administratively added to the router to block traffic only between user A's PC and server 1.

The following is an elaboration of Step 2 of the seven-step methodology.

Step 2 Establish a Theory of Probable Cause

This is the point in the troubleshooting process where your experience and intuition can be extremely helpful because it is when you brainstorm a list of possible causes. As you brainstorm, be sure to question the obvious. Also, think in terms of top-to-bottom troubleshooting (moving from top to bottom within the OSI model) or bottom-to-top troubleshooting. Alternatively, you could use a divide-and-conquer troubleshooting approach.

When examining your collected data (for example, output from ipconfig /all command), question everything. For example, you might think that the issue described in step 1 (identify the problem) could result from causes such as an ACL blocking traffic to or from the PC, a connectivity issue with the PC or server, or an incorrect IP address configuration on the PC.

From your list of possible causes, select the one you consider the most likely. From the previous list, you might believe that an incorrect IP address configuration on the PC is the most likely cause of the problem. Specifically, you might conclude that the issue is not related to connectivity because other PCs can get to the server, and user A's PC can get to other servers. Also, you might conclude that it is more likely that user A's PC has a bad IP address configuration than that an ACL has been administratively added to the router to block traffic only between user A's PC and server 1.

Elaboration of Step 1 and Step 3 though Step 7 of the network troubleshooting methodology that should be understood and memorized for the Network+ exam can be found in CompTIA Network+ N10-008 Cert Guide

About The Author

Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, StormWind.com. Anthony is also a VMware Certified Professional.

CompTIA Network+ N10-008 Cert Guide contains proven study features that allow you to succeed on the exam the first time. Expert instructor Anthony Sequeira shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills, essential for successful completion of the performance-based testing items on the exam. This complete, CompTIA-approved study package includes the following:

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Remote access
Network troubleshooting

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