show mac address table on Fortigate

The command is 'get sys arp'. It will show you all learnt arps on the FortiGate with the interface that learnt them.

If you'd like to quickly filter the results by portX then you can pipe a grep after the command (ie: get sys arp | grep portX)

In TP mode, we can check L2 forwarding table on FGT. 

The forwarding database (FDB) is populated with the network devices MAC addresses during a MAC learning process, based on the source addresses seen in the Ethernet frames ingressing a FortiGate port.

FGT # diagnose netlink brctl list  -> List Bridge information
list bridge information
1. root.b fdb: size=256 used=6 num=7 depth=2 simple=no
2. mgmt.b fdb: size=256 used=5 num=4 depth=2 simple=no
Total 2 bridges
Here above we can see two bridge instances for 2 VDOMs in Transparent mode: root and mgmt.

This command will dump the L2 forwarding table for each VDOM bridge instance:
diagnose netlink brctl name host <VDOM_name>.b

Example for the root VDOM:
FGT# diag netlink brctl name host root.b

show bridge control interface root.b host.
fdb: size=256, used=6, num=7, depth=2, simple=no
Bridge root.b host table
port no device devname mac addr ttl atributes
2 7 wan2 02:09:0f:78:69:00 0 Local Static
5 6 trunk_1 02:09:0f:78:69:01 0 Local Static
3 8 dmz 02:09:0f:78:69:01 0 Local Static
4 9 internal 02:09:0f:78:69:02 0 Local Static
3 8 dmz 00:80:c8:39:87:5a 194
4 9 internal 02:09:0f:78:67:68 8
1 3 wan1 00:09:0f:78:69:fe 0 Local Static

While the above may work for some, I couldn't get any useful information from it on an 8-port Fortiswitch. The bigger issue for me is that this seems to be treated like a fringe topic rather than a mainstream thing. Checking the mac table in a Cisco switch is a routine thing to do when you have a cause where a connected device that should be pulling dhcp will not. 

Hope this helps for future searchers:

diagnose lldprx nei sum