|  | 7.3 | Troubleshooting Networks
| 7.3.2 | Troubleshooting methods
The Process of Elimination and Divide and Conquer techniques are the most successful methods for networking troubleshooting. The following scenarios explain these techniques. The Process of elimination technique will be applied to the following problem: A user on your network calls the help desk to report that their computer can no longer get to the Internet. The help desk fills out the error report form and forwards it to you, the network support department. You call and talk to the user and they tell you that they have done nothing differently than they have always done to get to the Internet. You check the hardware logs for the network and find out that the user's computer was upgraded last night. Your first solution is that the computer's network drivers must be incorrectly configured. You go to the machine and check the network configuration information on the computer. It seems to be correct, so you Ping the server on that subnet. It doesn't connect. The next solution is to check to see if the workstation cable is plugged in. You check both ends of the cable and then try Pinging the server again. Next you Ping 127.0.0.1, the loopback address for the computer. The Ping is successful, so that eliminates a possible problem between the computer, driver configuration, and the NIC card. You decide then that there might be a problem with the server for this network segment. There is another networked computer at the next desk, so you Ping the server's address and the result is successful. This eliminates the server, the backbone, and the server's connection to the backbone as the problem. You then go to the IDF and switch the port for the workstation, go back to the workstation and try to Ping the server again. The solution still does not work. This narrows your search down to the horizontal cabling or the workstation patch cable. You go back to the IDF, put the cable back in the original switch port, get a new workstation patch cable and return to the workstation. Replace the workstation cable, and try to Ping the server again. This time it is successful, so you have fixed the problem. The last step is to document the problem solution on the error report form and return it to the help desk so it can be logged as completed. The Divide and Conquer technique will be applied to this problem: You are combining two networks that work fine when not connected, but when they are joined the entire combined network fails. Refer to the diagram for reference. The first step would be to divide the network back into two separate networks and verify that the two still operate correctly when separated. If this is true then remove all of the subnet connections for one of the connecting routers and reconnect it to the other working network. Verify that it is still working correctly. If the network is still functioning add each of that router's subnetworks back into the router until the overall system fails. Remove the last subnet that was added and see if the whole network returns to normal operation. If the network is again functioning normally, remove the hosts from the network segment and replace them one at a time, again checking to see when the network fails. When you find the offending device, remove it and verify that the network returns to normal. If the network is still functioning normally, you have isolated the faulty piece of equipment. It is now possible to troubleshoot this individual piece of equipment to find out why it was causing the entire network to crash. If nothing proves to be wrong with this device upon analysis, it may be that this device in conjunction with another device on the opposite network is causing the problem. To find the other end of the problem you would have to repeat the process that was used above. This is that process: First reconnect the host that caused the network to fail. Then disconnect all of the subnetworks from the other router. Check that the network has returned to operating status. If the network is functioning again, add each of that router's subnetworks back into the router until the overall system fails. Remove the last subnet that was added before the failure and see if the whole network returns to normal operation. If the network is again functioning normally, remove the hosts from the network segment and replace them one at a time, again checking to see when the network fails. When you find the offending device, remove it and verify that the network returns to normal. If the network is still functioning normally, you have isolated the other faulty piece of equipment. It is now possible to troubleshoot this individual piece of equipment to find out why it was causing the entire network to crash. If nothing proves to be wrong with this device upon analysis, compare the two hosts and find the reason for their conflict. By resolving this conflict, you will be able to reconnect both stations into the network and it will still function normally.
| Web Links
| Troubleshooting Overview
| Troubleshooting Tools
| Troubleshooting Hardware
| Troubleshooting Ethernet
| Troubleshooting TCP/IP
| Troubleshooting Serial Lines