4.9. A Troubleshooting Strategy for Network Problems
What seems like common sense in regard to troubleshooting is not as commonly applied as you might imagine. Often network professionals address problems in an unorganized manner. First, they try to determine everything that could possibly be wrong with the network, then they make every change that they assume is warranted, and finally they test the results. This approach leads at best to a solution that cannot be defined or documented, and at worst leads to endless frustration over continually fixing and then rebreaking the network. There is, however, a much better method of troubleshooting a network problem. In this section, we discuss the steps involved in this method and how they can be applied to correct a network problem.
4.9.1. Critical Information
Given a network problem scenario, you should be able to select an appropriate course of action based on a logical troubleshooting strategy. We will discuss the details of the logical troubleshooting strategy that you should know.
4.9.1.1. Troubleshooting Strategy
An effective network troubleshooting strategy should do more than fix a network problem; it should define the cause of the problem and document the solution to the problem. For this to be possible, you must know what action fixed the problem when you are done. This means that you have to isolate a potential cause, make a change, and test repeatedly until the problem no longer exists. To be more specific, let's say that a user cannot connect to the Internet. You want to start troubleshooting the problem to help the user get connected. You should proceed through the following steps:
Identify the symptoms and potential causes. In this case, the symptom seems simple: the user cannot access the Internet. Potential causes could be a problem regarding the user's browser, IP address, subnet mask, default gateway, DNS address, and so on. It could also be a problem with the ISP.
Identify the affected area. In other words, which other users also cannot access the Internet? Can this user access internal websites with his browser? Can this user ping the address of other clients and servers in the network?
Establish what has changed. In this case, there are many questions that you could ask. Has there been a recent change in ISPs? Has anyone else used the user's computer in the recent past? Has anyone added or deleted any software from the computer between the time that the user could access the Internet and now? For that matter, could this computer ever access the Internet?
Select the most probable cause. Suppose that you determine that some other users also cannot access the Internet, but not all users. Furthermore, this user can access the internal web-sites with his browser. Also, this user can ping other clients in the network by their IP address as well as their hostname.
If this were the case, then it would seem that a DNS address for the servers that lead to the Internet might be at fault. Next, you determine that the ISP the company uses has changed very recently. Could it be possible that the DHCP servers are still configured to assign the old DNS address to clients?
Implement an action plan and solution, including potential affects. You decide to check the scope options of the DHCP server to determine the DNS address that the DHCP servers are assigning. If the DHCP servers are assigning the old address, this could be the problem. You find that the new DNS address is in the scope, so this is probably not the problem. Next, you check to make sure that the clients that cannot access the Internet have renewed their addresses with the DHCP server since the date of the ISP change, and you find that they have not.
Your action plan, therefore, is to release and renew the IP addresses of the clients that cannot access the Internet. If they are configured with an invalid DNS server address, this should fix the problem and allow them to connect.
Test the result. You release and renew the addresses on the clients and then attempt to access a website on their browser. The attempt is successful and the client is restored access to the Internet.
Identify the results and effects of the solution. In this case, the solution is a permanent one for the clients that were affected (at least until the ISP changes again). You may also want to make sure that all of the other clients have renewed their address, and force a renewal on any clients that haven't, before they find that they can't access the Internet either.
Document the solution and process. Now that you have solved the problem, you should take a moment to document the entire sequence of events, including the symptoms of the problem, what you did to fix it, and what could be done in the future to prevent it from occurring again. This information might be very useful in the future for you or another network professional.
4.9.2. Exam Essentials
Know how to perform logical troubleshooting. An effective network troubleshooting strategy not only fixes the problem but also identifies its true source and documents the results. You should be able to apply the eight steps of troubleshooting strategy to a network troubleshooting strategy.