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I was changing out the ethernet cable on a running 6.0 machine (upgraded from 5.2) and when the new cable was in (very fast switch), I had no traffic across eth0. ifconfig looked fine and was up. Lights on the nic and the hub, no traffic lights. ifconfiged eth0 down and up, no go. Did a 'reboot' and it still was not passing traffic across the port. Did a 'shutdown -h now', then brought it up and was fine and passing traffic again.
This sounds like a hardware problem.
Some (poorly designed, IMHO) cards do this. It is almost certainly a hardware issue. Possible workaround: rmmod followed by insmod. Generally, forcing the driver to reinitialize the card will solve this, but there are some *really* bad cards out there. The chipset issue is usually something like: - cable removed - chipset detects no ethernet carrier; automatically tries to renegotiate 10/100/HD/FD/whatever/TP/coax/ whatever it does; - fails to find an ethernet connection; locks into some default mode that may not be supported. Default mode is often 10b2 or 10b5 (coax and AUI, respectively) -- but most cards don't have these physical ports.