Chapter 10 Install and System Administration for FortiOS 5.0 : Advanced concepts : Dual internet connections (redundant Internet connections)
  
Dual internet connections (redundant Internet connections)
Dual internet connection, dual WAN, or redundant internet connection refers to using two FortiGate interfaces to connect to the Internet. Dual internet connections can be used in three ways:
redundant interfaces, should one interface go down, the second automatically becomes the main internet connection
for load sharing to ensure better throughput.
a combination of redundancy and load sharing.
Video: Redundant Internet connections using ECMP
See Also
Redundant interfaces
Load sharing
Link redundancy and load sharing