Chapter 17 System Administration : Advanced concepts : Video: Redundant Internet connections using ECMP : Link redundancy and load sharing
  
Link redundancy and load sharing
In this scenario, both links are available to distribute Internet traffic over both links. Should one of the interfaces fail, the FortiGate unit will continue to send traffic over the other active interface. Configuration is similar to the Redundant interfaces configuration, with the main difference being that the configured routes should have equal distance settings.
This means both routes will remain active in the routing table. To make one interface the preferred interface, use a default policy route to indicate the interface that is preferred for accessing the Internet. If traffic matches the security policy, the policy overrides all entries in the routing table, including connected routes. You may need to add a specific policy routes that override these default policy routes.
To redirect traffic over the secondary interface, create policy routes to direct some traffic onto it rather than the primary interface. When adding the policy route, only define the outgoing interface and leave the gateway blank. This ensures that the policy route will not be active when the link is down.
See Also
Redundant interfaces
Load sharing
Dual internet connections (redundant Internet connections)