off brand routers and switches don't handle a lot of odd but allowed things.

I ran into this on some managed D-Link gigabit switches. It was driving me nuts as the machine in question was the Business owners custom made machine.

I ended up having to disable the NIC on the motherboard and put in a new NIC. It would not take an assignment of a MAC address.

When I saw this thread... and read the first message, my first thought was all zero MAC addr. Its a bugger to troubleshoot. You almost have to sniff the connection, but anything you put in between the computer and the switch *fixes* or completely *masks* it, typically.