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May the source be with you, but remember the KISS principle ;-)
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Those days this is are problem. So it is pretty difficult to diagnose when it does happen. You need to differenciate faulty cable from falty connection.
Often faulty cable problem develops because the plastic lock fall out. Then connection can became unreliable due to vibration. This is more typical for home networks for obvious reasons, but can sometimes happen in datacenters too.
Check that the link status LED is lit first. Then test the connection with your laptop or with a known working cable. The link LED will be lit even if the transmit line is damaged. Verify that a mdi-x connection or crossover cable is being used if connecting hub to hub.
For example users on network A could not reach hosts on network B even though routers R1 and E2 appeared to be functioning normally.
First you need to verify that the routers R1and R2 were configured correctly and that the interfaces are up. They you need to verify that systems A and B were up and configured correctly.
You need to use the traceroute utility to discover the actual route from system A to system B.
For example the traceroute output shows that the attempted route from system A on network net-1 goes through router R1 as expected. But the traffic never reaches router R2 though.
Investigate the router R2 log files. For example is they show that the interface to network net-2 is flapping (going up and down at a very high rate) and corrupt routing tables you can suspect that the cable is a problem.
To solve this problem, replace the network net-2 cable to router R2. If it fixes the problem then it was faulty and causes intermittent connections.
Troubleshooting a cable problem
Although many types of cables are used for networking, the most commonly used fall into two categories: coax (which looks similar to the wire used for cable television) and twisted pair (which is similar to telephone cable). Because these types of cable are so dissimilar, they require different troubleshooting methods.
Troubleshooting twisted pair
Twisted pair is considerably easier to troubleshoot than coax. Because of the nature of twisted pair, each line services only one PC (unless that line happens to run between two switching devices). If you suspect a cable problem in a network that uses twisted pair, the first thing you should check is the link light. If the link light isn't lit, it means that you don't have a complete physical link. Usually this indicates a break in the cable or a loose RJ-45 connector. Note that the link light can sometimes be illuminated even if you have a cable problem: link light merely indicates that the wire is connected at both ends.
If the link light is lit but you still suspect a cable problem, you should plug a known good laptop computer into the cable. If the laptop establishes a network connection, the problem is with your PC. Otherwise, you have a cable problem.
The sidebar "Coax Basics" provides some background on coax-based networks. Needless to say, most cable problems on coax-based networks affect multiple PCs. If you have a communication failure but your terminators are connected and are the correct type, you should check for a cable break. A break in the cable causes the wire to function as two separate unterminated networks, because the point at which the cable ends (the break) is unterminated.
Breaks or shorts in coax cables are often hard to find, because they aren't always visible to the naked eye. A break could be caused by something as simple as a loose T-connector. Although the wire may not be completely pulled out of the connector, it may be loose enough that it can't make a good connection. It's also tricky to locate breaks and shorts when a coax cable snakes its way through walls and conduit, under desks (where users often stack boxes on top of the wire), and into other inaccessible places.
The easiest way to troubleshoot a coax segment is to take two known good terminators and use one to terminate the cable at the source. The source is the place at which the cable connects to the server, another segment, a hub, and so on. Go to the first PC on the segment and disconnect the T-connector from the PC. Remove the portion of the line that goes to the rest of the network and replace it with the second terminator. Now, reconnect the T-connector to the PC and try to log on to the network with the PC. If the PC fails to connect, your problem is somewhere between the two terminators.
If the PC does connect, remove the terminator from the T-connector on the PC, reattach the cable to the T-connector, and repeat the process at the second PC on the line. As you get further down the line, you'll reconnect and test one PC at a time until you come to the source of the problem. If you fix a problem but the line still malfunctions, keep in mind that you may have multiple breaks on the line.
Occasionally, you may trace a problem to a particular PC and yet be unable to find a break in the line. If this happens, try disconnecting the T-connector from that PCbut enable the rest of the PCs on the line and test the network. Sometimes, a network card will go bad and flood the line with high-volume random packets. If this happens, it can cause symptoms similar to a cable break or a missing terminator.
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Troubleshooting network problems (Microsoft Technet).
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PC Repair Guide Network Interface Card (NIC) Diagnosis, Repair and Replacement - ACME HOW TO.com
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