I just installed a used motherboard in my tower, booted up and everything worked EXCEPT ethernet. Lspci tells me everything but the cpu is nvidia mcp61. I thought, OK, I'm on Debian Jessie so this is probably a missing proprietary driver. Or maybe my install is just freaking out because I swapped motherboards.

So I tried several live discs (elementary, xubuntu, linux lite, sparky) and none of them had working ethernet either. Hmm.... this made me wonder if I have a bad ethernet jack.

Next, I googled nvidia mcp61 and found lots of posts about problems with audio and video and one post about non-working ethernet on Debian 6. The only answer that guy got was that it "should" work :-(

Not sure what I should do next. Is nvidia mcp61 known to cause problems on linux? Or do I have a dead ethernet jack? What is the quickest way to verify if the ethernet actually works?

It's a very common mobo fault - dead ethernet chip. Just buy any cheap PCI or PCI-Express Ethernet card and use it instead of dead built-in card.

Though you have to check BIOS option for ethernet - it may be disabled. If no - buy new one.

Awesome. Thank you. I'll go thru the BIOS again and look for an ethernet switch.