What is the preferred interface setting on Digium gateway appliances?

Preferred Interface is a setting used to set the default interface that packets will use in order to reach other networks. This option will be ignored on TCP connections* or static routes. 

On G400 and G800 gateways, which have two ethernet interfaces, enabling both interfaces with the setting Preferred Interface is not supported, and will result in adverse effects when attempting to correctly route traffic.

* TCP connections are stateful therefore any response to a TCP request will be done on the same interface that the request was received.