MacBook won't pick up networking on ethernet

Solution 1:

As you have tested MBP2 in another network and it worked well, and as MBP1 works without issues, it might just be that your switch is a bit off and MBP2 too, so that together they simply do not get along but still work fine with other partners.

One possible test might is to try another switch, loaned from a friend or a shop, and see if this solves the problem. If it does, then the switch is the problem.

Another test is to use a USB network card on MBP2. If this solves the problem, you might be able to force Apple Store to replace your MBP2 card, or stay with the solution of the USB card.

Solution 2:

I've seen Ethernet interfaces begin having hardware failures by failing to establish link with some switches, especially at gigabit speed. When plugging my last MBP into the Cisco GigE switch at work (which it had worked with for years), it would keep cycling link up and down until I manually configured it for 100/Full. Leaving it set for full autonegotiation would work with other switches, especially 10/100 switches.

See if manually configuring your failing MBP's Ethernet speed/duplex settings for 100/Full fixes the problem connecting to the GigE switch.

If you can't get any connectivity over Ethernet and if a clean reinstall of Mac OS X doesn't fix it, then you have a hardware failure that needs repair.