It doesn't matter which one of the two Thunderbolt ports I use, neither of them work. I can't be 100% sure of this though, but it appears to recognise me attaching that adapter. I am more or less convinced that the Thunderbolt Port on the MBP is also working - when checking in System Report > Thunderbolt I can at least see the Thunderbolt Bus and a Thunderbolt to Gigabit Ethernet Adapter, which is listed as Device Connected. I also happen to have a second Thunderbolt Ethernet adapter and it displays the exact same issues. I am sure the Thunderbolt Ethernet adapter and network cables are working fine - when using a MacBook Air everything connects just fine.

I have tried various solutions that one can find online, but none of those worked. I am using a MacBook Pro Retina (Early 2013) and an AirPort Time Capsule (in the more recent 802.11ac tower version). Now, when opening the System Preferences under Network I only see the Status: Not Connected message` for the Thunderbolt Ethernet adapter. I assumed there was some odd bug somewhere, maybe triggered by certain network activity, but since I could solve it with a restart, it didn't make me concerned. My machine would simply drop from the network and not be able to get back on without a restart. Over the last few days I had already seen my ethernet connection 'crash' multiple times. It appears my Thunderbolt Ethernet connector has stopped connecting to my router for good today.

Air mac ethernet adapter