WHEN DID THE PROBLEM BEGIN TO OCCUR

3. When did the problem begin to occur? More specifically, you should find out what changed on the network when the issues first began. If your users began to experience connectivity issues after you installed a new router, your first step will be to check the configuration of the router. Sometimes this can take a certain amount of investigation, as you may not be aware of everything that’s being administered or modified on a large network. Your Internet Service Provider (ISP) might have made some changes that you’re not aware of, which might be caus-ing the connectivity failures.522ChApTEr 11: Network Troubleshooting Tools

Analyzing and responding to a problem

Now that you’ve collected information about the problem, you need to analyze all of it to determine the cause. You should examine every layer in the OSI model starting at Layer 1. Check your physical layer connec-tivity, such as cables, patch panels, wall jacks, and hubs. Work your way up to Layer 2, verifying that any switches or switch ports are configured properly. At Layer 3 you’ll verify that your routers are configured and func-tioning properly, and then move onto troubleshooting the actual application itself at the transport layer and beyond. In almost all cases, it’s best to start at the physical layer and work your way up so that you’re eliminating the lowest common denominator as a potential issue before moving onto more complex troubleshooting problems. If two computers aren’t communicating simply because a cable is unplugged, you can save yourself quite a bit of time by starting with the physical layer and working up from there.You’ll then want to determine the proper troubleshooting tool to use. We’ll spend the majority of this chapter talking about the different tools available for your use, but as a rule you should start with basic connectivity tools like ping, moving on to other tools once you’ve determined that basic connectivity is in place. The reason we start with ping is because it is simple to use and lets you determine at a glance if two nodes on your network are able to communicate with each other at the most basic level. It also very effectively slices the OSI model in half for you in terms of troubleshooting, because ping operates at the network layer. For example, if you are able to ping a remote host but you cannot File Transfer Protocol (FTP) to it, then you know that the issue lies somewhere above the network layer, because otherwise ping would not be successful. If the ping is unsuccessful, then you know that the issue lies in the network, data link, or physical layers.In Exercise 11.1, we’ll look at a common situation where you might use ping to investigate a connectivity problem between two computers. Let’s say that you’re troubleshooting a connectivity problem between Computer A, with an IP address of 192.168.1.1 and subnet mask of 255.255.255.0, and Computer B with an IP address of 192.168.2.5 and a subnet mask of