Identifying and Testing the Cause of the Problem

      After you develop a theory about the cause of the problem, test your theory. The test must conclusively prove or disprove your theory.

Two general rules of troubleshooting are:

• If you cannot reproduce a problem, then no problem exists unless it happens again on its own.
• If the problem is intermittent and you cannot replicate it, you can configure your network management software to catch the event in progress.

      For example, with"LANsentry Manager", you can set alarms and automatic packet capture filters to monitor your network and inform you when the problem occurs again. See"Configuring Transcend NCS" for more information.

      Although network management tools can provide a great deal of information about problems and their general location, you may still need to swap equipment or replace components of your network until you locate the exact trouble spot.

      After you test your theory, either fix the problem as described in"Solving the Problem" or develop another theory.

Sample Problem Analysis
       This section illustrates the analysis phase of a typical troubleshooting incident. On your network, a user cannot access the mail server. You need to establish two areas of information:

• What you know - In this case, the user's workstation cannot communicate with the mail server.
• What you donot know and need to test-
• Can the workstation communicate with the network at all, or is the problem limited to communication with the server? Test by sending a"Ping" or by connecting to other devices.
• Is the workstation the only device that is unable to communicate with the server, or do other workstations have the same problem? Test connectivity at other workstations.
• If other workstations cannot communicate with the server, can they communicate with other network devices? Again, test the connectivity.

The analysis process follows these steps:

1. Can the workstation communicate with any other device on the subnetwork?• Ifno, then go to step 2.
   • Ifyes, determine if only the server is unreachable.
   • If only the server cannot be reached, this suggests a server problem. Confirm by doing step 2.
   • If other devices cannot be reached, this suggests a connectivity problem in the network. Confirm by doing step 3.
2. Can other workstations communicate with the server?
   • Ifno, then most likely it is a server problem. Go to step 3.
   • Ifyes, then the problem is that the workstation is not communicating with the subnetwork. (This situation can be caused by workstation issues or a network issue with that specific station.)
3. Can other workstations communicate with other network devices?• Ifno, then the problem is likely a network problem.
   • Ifyes, the problem is likely a server problem.

When you determine whether the problem is with the server, subnetwork, or workstation, you can further analyze the problem, as follows:

• For a problem with the server - Examine whether the server is running, if it is properly connected to the network, and if it is configured appropriately.
• For a problem with the subnetwork - Examine any device on the path between the users and the server.
• For a problem with the workstation - Examine whether the workstation can access other network resources and if it is configured to communicate with that particular server.

Equipment for TestingTo help identify and test the cause of problems, have available:

• A laptop computer that is loaded with a terminal emulator, TCP/IP stack, TFTP server, CD-ROM drive (to read the online documentation), and some key network management applications, such as LANsentry Manager. With the laptop computer, you can plug into any subnetwork to gather and analyze data about the segment.
• A spare managed hub to swap for any hub that does not have management. Swapping in a managed hub allows you to quickly spot which port is generating the errors.
• A single port probe to insert in the network if you are having a problem where you do not have management capability.
• Console cables for each type of connector, labeled and stored in a secure place.