Hello
Once you create sub interfaces then you would probably require to associate a dot1q tag (vlan) to it.
int x/x
ip address 10.10.10.1 255.255.255.0 < default will be native vlan 1
int x/x.2
encapsulation dot1q 2
ip address 10.20.20.1 255.255.255.0 < tagged vlan id 2
So from the example above the switch connecting to the FTD its switchport needs to be in vlan 2, if the FTD has ip address on the physical & the sub-interface than the same switchport needs to be a trunk port.
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Kind Regards
Paul