Sometimes you want to test a switch's trunk port without having to carve our an access port to plug your MacBook into.

Here's how:

1. Open System Preferences
2. Click Network
3. Select your Thunderbolt Ethernet adapter
4. Click the gear below the sidebar listing the interfaces
5. Click Manage Virtual Interfaces
6. Click the + button
7. New VLAN
8. Enter a name, such as "VLAN-200"
9. Enter a VLAN ID in the Tag field, such as 200
10. Choose Tunderbolt Ethernet for your Interface
11. Click Create
12. On the sidebar, your new vlan ("VLAN-200") is now an option. Click it.
13. Configure as needed.