What is the nature of the packet drops? Is it full loss of packets or intermittent? How long do they last? Does all traffic get dropped at the firewall? Or just traffic going to a specific interface? i.e. can you still access certain interfaces on the firewall when the packet drops are happening? Please provide many more details so we can possibly help.
OK so the packet drops happen every 14-15 minutes and they last 14-15 minutes? That's an interesting pattern.....
If you suspect it's a switch issue, have you checked interface stats on both sides? Any drops, errors, etc?
Is the Catalyst 9300 connected to both LAN and DMZ interfaces or just one of those interfaces? Can you run a test where you do continuous pings from a host behind the Catalyst to one of the directly-connected FGT interfaces? So if LAN interface is connected to Catalyst ping LAN interface from host behind Catalyst. Do packet drops occur? This should pretty much help to rule out the Catalyst
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