MAC address of zero
 

I have an older x86 processor (Pentium III Coppermine) running a 2.6.23.17 kernel. It has an ethernet interface and a USB (among other things). The kernel loads both the e100 and eepro100 modules -- that's all I know about the specific ethernet interface hardware. The ethernet interface is connected to an ethernet switch. It normally boots up just fine.

Sometimes I connect one or more RTL8150 USB<->Ethernet chips between the processor's USB and the same ethernet switch. (I know that normally doesn't make sense. I can explain if you need to know.) When I plug in the RTL8150's, sometimes the built-in Ethernet chip boots up with a MAC address of zero. The networking software appears to think nothing is wrong; ifconfig reports the interface as (statically) configured and up. But the rest of the universe seems to drop all packets from this interface when its MAC address is zero. Or maybe it won't actually send anything when it's in this state.

For a while I thought the built-in ethernet chip's MAC address EEPROM might be flakey, but so far it's 100% reliable when the RTL8150's are absent.

When it decides to have a MAC address of 00:00:00:00:00:00, it seems to persist across power cycles until I remove the RTL8150's. Once it's booted up correctly, I can turn off power, reinsert the RTL8150's exactly as they were, and boot up, and chances are it will be fine again.

I can obtain more detailed information about the built-in ethernet chip and the switch, if you like.

Any suggestions as to what might be going on?

Try plugging the USB NIC into a router with a different subnet.