diagnose hardware deviceinfo nic <interface-name> (number of packets dropped by an interface)

This command displays a wide variety of statistics for FortiGate interfaces. The fields Host Rx dropped and Host Tx dropped display the number of received and transmitted packets that have been dropped.

diagnose hardware deviceinfo nic port2
...
============ Counters ===========
Rx Pkts         :20482043
Rx Bytes        :31047522516
Tx Pkts         :19000495
Tx Bytes        :1393316953
Host Rx Pkts    :27324
Host Rx Bytes   :1602755
Host Rx dropped :0
Host Tx Pkts    :8741
Host Tx Bytes   :5731300
Host Tx dropped :0
sw_rx_pkts      :20482043
sw_rx_bytes     :31047522516
sw_tx_pkts      :19000495
sw_tx_bytes     :1393316953
sw_np_rx_pkts   :19000495
sw_np_rx_bytes  :1469318933
sw_np_tx_pkts   :20482042
sw_np_tx_bytes  :31129450620