The Divide-and-Conquer Troubleshooting Approach
The divide-and-conquer approach to network troubleshooting, unlike its top-down and bottom-up counterparts, does not always commence its investigation at a particular OSI layer. When you apply the divide-and-conquer approach, you select a layer and test its health; based on the observed results, you might go in either direction (up or down) from the starting layer. Figure 6-3 depicts the divide-and-conquer troubleshooting approach. If a layer is in good working condition, you inspect the layer above it. If a layer is not in good working condition, you inspect the layer below it. The layer that you ultimately select as the first targeted layer is the one that is faulty, and the layer below it is in good working condition. The particular layer at which you begin the divide-and-conquer approach is based on your experience level and the symptoms you have gathered about the problem. For example, if a user reports that he can't go to or has some trouble with a particular Web page but has no trouble going to or using other Web pages, you can safely decide that you do not need to begin troubleshooting at the physical, data link, or even the network layer. However, if many users report that they have problems accessing all resources on the Internet, you might start at the network layer and take the next step based on those findings.
Figure 6-3 A Divide-and-Conquer Troubleshooting Approach
During the course of divide-and-conquer troubleshooting, if you can verify that a layer is functioning well, you can pretty safely assume that the layers below it are functioning as well. If a layer is not functioning at all or it is working intermittently or erroneously, you must immediately inspect the layer below it (with the exception of the physical layer, which does not have a layer below it). If the layer below the current layer is in good working condition, the culprit resides in the current layer. If the layer below is also malfunctioning, you should gather symptoms of the problem at that layer and work your way down.