Using the layered models, there are three primary methods for troubleshooting networks:
- Bottom-up
- Top-down
- Divide-and-conquer
Each approach has its advantages and disadvantages. This topic describes the three methods and provides guidelines for choosing the best method for a specific situation.
Bottom-Up Troubleshooting Method
In bottom-up troubleshooting, you start with the physical components of the network and move up through the layers of the OSI model until the cause of the problem is identified, as shown in Figure 1. Bottom-up troubleshooting is a good approach to use when the problem is suspected to be a physical one. Most networking problems reside at the lower levels, so implementing the bottom-up approach is often effective.
The disadvantage with the bottom-up troubleshooting approach is it requires that you check every device and interface on the network until the possible cause of the problem is found. Remember that each conclusion and possibility must be documented so there can be a lot of paper work associated with this approach. A further challenge is to determine which devices to start examining first.
Top-Down Troubleshooting Method
In Figure 2, top-down troubleshooting starts with the end-user applications and moves down through the layers of the OSI model until the cause of the problem has been identified. End-user applications of an end system are tested before tackling the more specific networking pieces. Use this approach for simpler problems, or when you think the problem is with a piece of software.
The disadvantage with the top-down approach is it requires checking every network application until the possible cause of the problem is found. Each conclusion and possibility must be documented. The challenge is to determine which application to start examining first.
Divide-and-Conquer Troubleshooting Method
Figure 3 shows the divide-and-conquer approach to troubleshooting a networking problem. The network administrator selects a layer and tests in both directions from that layer.
In divide-and-conquer troubleshooting, you start by collecting user experiences of the problem, document the symptoms and then, using that information, make an informed guess as to which OSI layer to start your investigation. When a layer is verified to be functioning properly, it can be assumed that the layers below it are functioning. The administrator can work up the OSI layers. If an OSI layer is not functioning properly, the administrator can work down OSI layer model.
For example, if users cannot access the web server, but they can ping the server, then the problem is above Layer 3. If pinging the server is unsuccessful, then the problem is likely at a lower OSI layer.