FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
The purpose of this document is to explain the split port capability on the FortiController 59x3C
FortiController 5.2

The FortiController 5903C/5913C supports the split port feature:
  -  5903C has 4 40G ports. Each 40G port can be split in 4x10G.

  -  5913C has 2 100G ports. Each 100G port can be split in 10x10G.

The following command can be used to enable the split port:
# conf sys global
(global)# set fabric-front-port-10g-mode enable
WARNING: Enabling fabric channel front port 10G mode will clear the existing interface configuration and reboot system.
Do you want to continue? (y/n)y
(global)# end
The system is going down for reboot. Please wait…

If the Fortigate worker blades were already in FortiController mode, they will get the parent and the split interfaces. When a FortiGate worker joins the SLBC, the FortiController advertises the front port list through the management heartbeat. These interfaces are added to the FortiGate configuration.

Note: If the split port is not enabled on the FortiController end, the FortiGate will add fctrl/f1 and fctrl/f2 to the configuration (in case of a FortiController 5913C).

Note: If the split port is enabled on the FortiController, the FortiGate will add fctrl/f1-1, fctrl/f1-2 ... fctrl/f1-10, fctrl/f2-1 ... fctrl/f2-10, but the parent interfaces (fctrl/f1 and fctrl/f2) will not be deleted. These interfaces will remain in the FortiGate configuration, causing internal mapping conflict upon reboot.

Warning : In order to avoid such issues, the Fortigate workers must be factory reset.
FG-5KD3915800467 (global) # exe factoryreset
This operation will reset the system to factory default!
Do you want to continue? (y/n) y

Related Articles

Technical Note: FortiController 5903C-5913C split port capability