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I have a thunderbolt to ethernet cable on a MacBookPro Retina on Lion.

Sometimes the thunderbolt to ethernet connection does not work. It seems to occur when I switch network locations (e.g., I move from my home network to my work network). Restarting the computer fixes the problem, however, I don't want to have to restart my computer.

How can I force thunderbolt to ethernet to connect without restarting the computer?

More details:

Under network settings, there is a red dot and not connected on "Thund...thernet". It further says:

Status: Cable Unplugged
Either the cable for Thunderbolt Ethernet is 
not plugged in or the device at the other end 
is not responding.

Under system information - thunderbolt it states

Thunderbolt to Gigabit Ethernet Adapter:

  Vendor Name:  Apple, Inc.
  Device Name:  Thunderbolt to Gigabit Ethernet Adapter
  Vendor ID:    0x1
  Device ID:    0x8003
  Device Revision:  0x1
  Route String: 3
  Firmware Version: 5.5
  Port:
  Status:   Connected
  Link Status:  2
  Port Micro Firmware Version:  0.0.8

Under Network - Thunderbolt Ethernet it states:

Thunderbolt Ethernet:

  Type: Ethernet
  Hardware: Ethernet
  BSD Device Name:  en1
  IPv4:
  Configuration Method: DHCP
  IPv6:
  Configuration Method: Automatic
  Proxies:
  Exceptions List:  *.local, 169.254/16
  FTP Passive Mode: Yes
  Service Order:    0
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2 Answers 2

It seems like this issue at least for me may have had nothing to do with the thunderbolt connection. Rather it was a more mundane networking issue. I.e., when I moved between networks, network settings needed to be reconfigured. Techniques like renewing DHCP lease and using the "assist me" procedure in network preferences seemed to be sufficient for me to reconnect using Ethernet after switching networks.

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I had this problem, too, and an Apple Support call and Genius Bar visit later, I had the answer: if you're using Network Locations that preexist your Thunderbolt display, it doesn't seem to auto-add the possibility of "Display Ethernet" to your available network ports. To solve this, either:

Create a new Location in your Network system preferences. It will finally detect the "Display Ethernet" and use it. It'll slowly change from disconnected to No IP to green-is-for-go.

or

Fix an existing Location by clicking the + to add a port and choosing Display Ethernet. It'll slowly change from disconnected to No IP to green-is-for-go.

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