Q My Mac Pro has two Ethernet ports, labelled 1 and 2. When sharing my Internet connection, I am offered several different network ports. Which should I select, and is there a way that I can use both my USB 3G modem and a secondary broadband connection from an Ethernet port as a backup?
A It does not matter which of the two Ethernet ports on a Mac Pro that you connect your network (or other cables) to: both work the same, but you will often find port 1 referred to as en0, and port 2 as en1.
In the Sharing pane, when you configure Internet sharing you need to select first the connection that you wish to share. For an Ethernet connection, this would be Ethernet 1 or 2, depending on which port is hooked up; for a USB 3G modem this will be labelled distinctively. Below that you need to specify the connection over which access will be shared, typically an Ethernet connection that is not being used for the Internet.
Currently, to share your USB 3G modem access, you will need to hook other computers up via an Ethernet cable (or other network connection), and select that in the lower part of the pane.
To add a broadband modem-router, connect it to the free Ethernet port on your Mac Pro so that you can continue sharing from your primary 3G connection. If you put the modem-router onto your network, you will still be able to share the 3G connection, but your Mac Pro and other computers will choose which connection they use in the Network pane instead, where you should set up different locations to enable quick switching between 3G and the secondary broadband.
First ensure that both Ethernet ports are connected to networks which have separate Internet access, and both are activated and connected in the pane. Open the Action popup menu at the foot of the list of network interfaces, and select the Manage Virtual Interfaces… command. In the pane which appear, click on the + tool to create a New Link Aggregate…, and then select the two Ethernet ports in the next pane. Give the aggregate a new name, and Apply the changes. You should then be able to use the two ports as if they were one. OS X uses the standard Link Aggregation Control Protocol (LACP) for this purpose.
Updated from the original, which was first published in MacUser volume 26 issue 17, 2010.