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) didnt recognize 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 the computer was connected, was still shiny green. Same thing on the network card: 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 thats 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 found, 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, thats 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 netcard to be completely out - until I cut the electrical circuit of the computer by pulling the power cable.
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