Magical ethernet interface (network card not detected - electrical circuit issue?)

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Published on - Listed in Hardware Personal Network Rant

This weekend I have seen  the strangest network problem ever. For two weeks now, a network card in a selfbuilt computer using an Intel motherboard (NIC on board) has been defect. Even the operating system (SLES11) didn't detect it anymore. Yesterday I had to take a look at it in person and unfortunately the network card seemed dead. Dead for the OS but not for the switch; the LED on port 3, where this computer was connected to, was still shiny green. Same thing on the network card on the motherboard: The LEDs were green.

I tested several things like new configuration of ifcfg-eth0, repair install, new netcard detection... but that all doesnt work if even "hwinfo --netcard" shows no info. So I tried it with a cdboot-Linux (Knoppix 6.2). But not even Knoppix could find a network card; which means the problem is hardware-related. Before giving up and getting ready to replace the motherboard I wanted to check if all internal connections are ok. So I unplugged all cables from the computer, including the power cable. I was just about to open the computer when I saw the impossible: The LED for port 3 on the network switch was still a steady green!

Now THAT is very odd. No power, no network connection but the lights are on... Now I unplugged the network cable from the switch, releasing the "electrical circuit" from port 3. As expected the LED turned off. I plugged all the cables again to the computer and gave it another try - boot that thing. I also replugged the network cable to the switch. At the time of boot the switch LED turned on, starting to blink once in a while. Then I turned my head to the monitor where SLES11 was booting. And to my surprise the network card was detected, the configuration with static IP set and everything was up.

I have never seen a network problem like that, somehow completely deactivating the network card to work or even to be detected anymore, but hey, that's a first. Some electrical signal/discharge or whatever was between the switch and this computer causing the switch to freeze the port and the computers network card to be completely out - until I cut the remaining electrical circuit by pulling the network cable from the switch.

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