This guide will provide information on how to control interface priority.
How to setup interface priority.
Depending on your needs you can define the interface priority of your choice, UPLINK or UPLINK2.
From the SIteManager GUI under System-->UPLINK or UPLINK2
Choose between First or Second.
Set the priority of this UPLINK interface relative to the other UPLINK interface(s).
As long as its Internet link is up, an interface with "First" priority will always be used for outgoing connections. If the Internet link goes down, the SiteManager will fail-over to using the interface with "Second" priority.