If we are working in large scale corporate network infrastructure, being able to do a network troubleshooting is very essential in case we are facing a network connection problem that impact to a single client computer or affecting the whole operational corporate network globally.
When a computer experiences a network communications problem, there are obviously many possible sources of error. Troubleshooting network problems could lie in the TCP/IP protocol stack, it could be a problem with the data-link layer protocol, or it could even be a hardware problem such as a broken cable or a faulty network interface adapter.
To do network troubleshooting it would be better to have a case study. Let’s back to our previous article about network problem and below diagram is a scenario model of the networking environment. Having the blue print networking diagram will help in troubleshooting network problems.
Network Troubleshooting - Network Diagram
The following scenario will help us in troubleshooting network problems based on specific problems that might occur in a networking environment, problems that might occur to a single computer or problems that affect global corporate network communication. We can analyze the problems using some network troubleshooting approaches.
A single computer problem
Assume that a single computer in mining office (please refer to the above diagram) cannot connect to the network, how to start troubleshooting network problems like this? Troubleshooting network problems that cause the computers disconnected from the network resources can be done with the following steps.
1. Can the user logon and access the network resources?
a. NO. Can you use your admin account to logon and access the network resources?
i. Yes, then check if the user account is problem (Username and password are not correct; username is expired; or username is locked-out)
ii. No, proceed to step 2
b. YES, then no network problem.
2. Logon to the computer using local admin account and check the general “Internet Protocol (TCP/IP)” property if setting is “Obtain the IP address automatically” and “obtain DNS server address automatically”. If your network system uses DHCP server then this setting must be set to auto. You can also check the configuration using the “ipconfig /all” command on the command line prompt.
a. If the setting is wrong, then correct it to “obtain an IP address automatically” and “obtain DNS server address automatically”. Jump back to step 1.
b. If the setting is normal, then proceed to step 3.
3. Check the patch cable to the wall jack (refer to LAN cable diagram), is the patch cable cut or loose connection?
a. YES. If the patch cable is cut, replace with a good one. If the patch cable loose connection, plug it. Jump back to step 1.
b. NO. Check if the LAN led indicator is blinking. A NIC card in the computer commonly has 2 led indicators, power and LAN indicator. If both led indicators are blinking (usually green light), the NIC card is working normal and indicate no network problem. Is the NIC – network indicator blinking with green light?
i. YES, there should be no network problem.
ii. NO, proceed to step 4.
4. Logon with the local admin account and check the control panel if the LAN card is “disabled”?
a. YES, the LAN Card is “disable”. You can fix the problem by “enable” the LAN card and you can test it again to step 1.
b. NO. Proceed to step 5.
5. Replace the patch cable with a good one, can you logon and access the network resources now?
a. YES. Then you fix the problem.
b. NO. There are three possible reasons:
i. The NIC card is broken
ii. The switch port is faulty
iii. The LAN cable between the wall jack and the switch is faulty; it could be a rat attack.
6. If you have a LAN tester with you, you can test the RJ-45 port at the wall jack to see the network line. Is the network line is OK?
a. YES. If the network line is OK, then the NIC card is faulty. Replace the NIC card with the backup NIC and jump back to step 1.
b. NO. There are two possible reasons, the LAN cable faulty or the switch port is faulty.
7. Check directly to the switch port with the LAN tester or move the cable to the other port and check if the line is Ok now.
a. YES. You can mark the port with “broken” sign or isolate the port with the tape.
b. NO. You can confirm now that the underlying problem is the LAN cable between the wall jack and the switch port. You can arrange a technician to run a new cable.
8. Congratulation, you have done a network troubleshooting successfully.
A block of a building problem
Network troubleshooting to a block of a building that cannot access to the central office is easy to troubleshoot but sometimes longer time to fix. But if you apply redundant links between the two buildings, a single line failure will not cause the problem. But you still need to fix the broken link.
How can you come to a conclusion that there is a broken link between the buildings? At least you need to have network diagram for your network infrastructure (see the above network diagram) to help you localize the problem. If all of the computers in the mining building cannot access the network resources, there must be a link failure between the buildings. See that the DHCP, DNS and active directory servers and other resources are in the HR building; none of the computers in the Mining building receives the IP addresses and cannot resolve names.
If you apply redundant links between the buildings, the failure of a link will not disrupt the network links but a couple of milliseconds as soon as the STP protocols recover the links.
Now, how can you do network troubleshooting when the whole site cannot access to the outside world across the router?
Or, how can you do network troubleshooting when all the computers cannot access the internet services?
Or, how can you do network troubleshooting when no remote users can access the company network?
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Ki Grinsing
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