Link status changes between a SonicWall UTM appliance and Fritzbox modem/router

The event log may show multiple entries "Multi-Interface Link Down" on WAN interface links.  The events may appear frequently, sometimes at 15 minute intervals, or even less.

Some Fritzbox devices have an energy saving mode, such as "Green Mode", which aims to save power by reducing the link speed.  If the link speed changes on the interface the firewall may report the interface as down. 

There are two possible resolutions:

1. Disable "Green Mode", or other energy saving mode, and set the port speed to 1Gbps on the Fritzbox.
   
   
   
   
   
2. Connect an unmanaged switch between the Fritzbox and Firewall