31 Days Before Your CCNA Exam

Day 29 29Using Layers to TroubleshootYou should already have extensive experience troubleshooting network problems—whether in anactual work environment, in a lab environment, or a combination of both. By now, you have developedyour own troubleshooting methodology. Maybe you like to check the physical layer first. Isthe cabling correct? Are all interface status lights green? Maybe you like to ping everything togather information about where connectivity is lacking. Then you use the results of your connectivitytests to isolate problems and drill down deeper. Maybe you just intuitively search for solutions,using your past experience to guide.Regardless of your method, a systematic troubleshooting methodology can help you troubleshootproblems more efficiently and with better success. There are three main methods for troubleshootingnetworks using the layers of the OSI model:■■■Bottom up: Start with the physical components and move up through the layers until theproblem is isolated. Use this approach when the problem is suspected to be a physicalone. Most networking problems reside at the lower levels, so implementing the bottomupapproach often results in effective results.Top down: Start with the end-user application and move down through the layers untilthe problem is isolated. Use this approach for simpler problems or when you think theproblem is with a piece of software.Divide and conquer: Start by collecting user experience, documenting the symptoms,and then, using that information, make an informed guess at which OSI layer to startyour investigation. After you verify that a layer is functioning properly, assume that thelayers below it are functioning, and work up the OSI layers. If an OSI layer is not functioningproperly, work your way down the OSI layer model.To effectively troubleshoot network problems, take the time to select the most effective networktroubleshooting method. Today we are just reviewing the general methods used to troubleshootnetwork problems. In the coming days, we discuss troubleshooting in more detail as we exploreimplementation-specific situations in switching and routing technologies.Study ResourcesFor today’s exam topics, refer to the following resources for more study.Resource Chapter Topic Where to Find ItFoundational ResourcesCCNA Exploration Online Chapter 2, “Communicating The Platform for Section 2.1Curriculum: Network over the Network” CommunicationsFundamentals LANs, WANs, and Section 2.2InternetworksNetwork Addressing Section 2.5Chapter 3, “Application Making Provisions for Section 3.2Layer Functionality Applications and Servicesand Protocols” Application Layer Section 3.3Protocols and ServicesExamples