I think the missing element in this setup is the ISL between SW1 and SW2. If both switches were stacked one would immediately see that both LACP trunks would be operating with 2 ports each.
If one switch fails, the trunk from the active FGT would halfe in bandwidth but continue to work. If you need the complete bandwidth at all times, monitor those links to trigger a HA failover if one link fails.
As Graham said, you can not make a LAG between two independent L2 switches. You must either stack them (e.g. Catalyst stack-ring) or make a VPC (Cisco Nexus). If you only have two stand-alone L2 switches, the best you could do would be something like this:
This will give you some link redundancy and aggregation, but in case of e.g. the switch of the active firewall goes down/is rebooted, it will trigger a HA-failover.
You have to link the two switches to prevent loops in the network and have a path for the traffic that reaches one and needs to go through the other.
If you configure LACP on FortiGate you have to consider a point. Don't put the ports of both FortiGate units in one LACP group on the switch. You should add them to two different groups. If you do the setup as your design, FortiGate will detect different switches on the ports, and one of the ports will work and the other will not. But we have two FortiGate. will they both choose the same switch or the master will do it? I am not sure. If the two units activate the link on different switches STP should disable one.
For completeness sake - yes, as others have said, you CANNOT create an aggregate trunk to 2 separate Cisco switches WITH LACP (you would get an error on FGT side "mismatch aggregate ID" of a sort and one aggregate would not form) . On the other hand, you can create an aggregate if you disable LACP altogether, and form the aggregate statically. I can't vouch how good will it be, or will it work as expected, but aggregate will come up. I had to do it (unfortunately) in specific circumstances and it worked - Fortigate to 2 Cisco switches w/o LACP.
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