Below shows how to display the arp table in a Fortinet 200A device running FortiOS version 3.0 or higher.
Short Answer: Type “get system arp” from the CLI
You can just type “get sys arp” from the CLI of the Fortinet 200A to get the ARP table. If you are running a previous version of FortiOS you can try the other two following options to display the ARP table.
FortiOS version 2.8: diag net1 neighbor list
FortiOS version 2.5: get sys inter