This has been my problem for about 6 months. On my 2011 MBP, if I connect a working ethernet cable to my ethernet port, it will either do nothing or my internet connection will die totally, even when I'm on a strong wifi network.
More intriguing (frustrating), when I jump on a network connected to a simple switch, I have no internet connection. I.E
ping www.google.com gets request timeouts or unreachable hosts. But I can run
ifconfig en0 ..., then
ssh into another computer on the network, and from that
ssh terminal I can connect to the internet no problem. My 'network', by the way, is just my laptop and a desktop connected to a network switch. This happens on multiple switches from different companies.
I've reset everything in Network Preferences, but no help. Has anyone seen this behavior? If so, what can I do to fix it?