The Top-Down Troubleshooting Approach
As its name implies, when you apply a top-down approach to troubleshooting a networking problem, you start with the user application and work your way down the layers of the OSI model. Figure 6-2 shows the top-down troubleshooting approach. If a layer is not in good working condition, you inspect the layer below it. When you know that the current layer is not in working condition and you discover that a lower layer works, you can conclude that the problem is within the layer above the lower working layer. After you have discovered which layer is the lowest layer with problems, you can begin identifying the cause of the problem from within that layer.
You usually choose the top-down approach when you have reason to believe that the problem is most likely at the application or other upper OSI layers. Past experiences, new software installations, changes in user interface, or added security features are common reasons for believing that the reported problems are most likely user, application, or at least upper OSI layer-related. The top-down troubleshooting approach is usually most suitable for problems experienced by one person or only a few people; that is because lower layer (that is, network infrastructure) problems usually affect more than one person.
You usually take the top-down approach for simpler cases. The disadvantage to selecting this approach is that if the problem turns out to be more complex or happens to spring from lower-layer culprits (physical, data link, or network), you will have wasted time and effort on examining the user applications or upper OSI layer components. Furthermore, if you have internetwork expertise, you might not necessarily have the expertise to diagnose or correct application layer issues. Network engineers often examine the components that fall within their area of responsibility, and if those happen to be in good working condition, the problem is then referred to the workstation, server, or application expert.
Figure 6-2 A Top-Down Troubleshooting Approach