Chapter 10 Install and System Administration for FortiOS 5.0 : Interfaces : Redundant interfaces
  
Redundant interfaces
On some models you can combine two or more physical interfaces to provide link redundancy. This feature enables you to connect to two or more switches to ensure connectivity in the event one physical interface or the equipment on that interface fails.
In a redundant interface, traffic is only going over one interface at any time. This differs from an aggregated interface where traffic is going over all interfaces for distribution of increased bandwidth. This difference means redundant interfaces can have more robust configurations with fewer possible points of failure. This is important in a fully-meshed HA configuration.
An interface is available to be in a redundant interface if:
it is a physical interface, not a VLAN interface
it is not already part of an aggregated or redundant interface
it is in the same VDOM as the redundant interface
it has no defined IP address
is not configured for DHCP or PPPoE
it has no DHCP server or relay configured on it
it does not have any VLAN subinterfaces
it is not referenced in any security policy, VIP, or multicast policy
it is not monitored by HA
it is not one of the FortiGate-5000 series backplane interfaces
When an interface is included in a redundant interface, it is not listed on the System > Network > Interface page. You cannot configure the interface individually and it is not available for inclusion in security policies, VIPs, or routing.
See Also
Loopback interfaces
Interface settings
Administrative access
Wireless
Virtual Switch
Software switch
One-armed sniffer