Link aggregation (IEEE 802.3ad) allows you to combine two or more connected physical interfaces together to form an aggregated link. This new link has the bandwidth of all the links combined for example if we aggregate 2 links of 100mb +100mb =200mb. If a link in the group fails, traffic is automatically transferred to the remaining interfaces with the only noticeable effect being reduced bandwidth to 100 mb again in case if one link fails to accomplish your requirement aggregate 3 links of each 100mb so total bandwidth of link reaches to 300mbps . If any one of link fails. Bàndwidth will drop to 200mb which is modem bàndwidth requirement.
This is similar to redundant interfaces, with the major difference being that a redundant interface group uses only one link at a time, while an aggregate link group uses the total bandwidth of the functioning links in the group, up to eight (or more).
Some FortiGate models support the IEEE standard 802.3ad for link aggregation.
An interface can be an aggregate interface if it meets the following criteria:
It's a physical interface, not a VLAN interface or subinterface.
It's not already part of an aggregate or redundant interface.
It's in the same VDOM as the aggregated interface. Aggregate ports can't span multiple VDOMs.
It doesn't have an IP address and isn't configured for DHCP or PPPoE.
To create an aggregate interface - GUI:
Go to Network > Interfaces and select Create New, then Interface.
Enter the Name as Aggregate.
For the Type, select 802.3ad Aggregate.
If this option doesn't appear, the FortiGate doesn't support aggregate interfaces.
In the Interface Members field, click + to add interfaces. Select port 1,2 and 3
In the Addressing mode field, select Manual.
Enter the IP address for the port of X.X.X.X