Cluster Interfaces
For a cluster isolated to security modules within one Firepower 9300 chassis, you can assign both physical interfaces or EtherChannels (also known as port channels) to the cluster. Interfaces assigned to the cluster are Spanned interfaces that load-balance traffic across all members of the cluster.
For clustering with multiple chassis, you can only assign data EtherChannels to the cluster. These Spanned EtherChannels include the same member interfaces on each chassis; on the upstream switch, all of these interfaces are included in a single EtherChannel, so the switch does not know that it is connected to multiple devices.
You can use regular firewall interfaces or IPS-only interfaces (inline sets or passive interfaces).
Individual interfaces are not supported, with the exception of a management interface.