A quick check for a momentary interruption of the internet on FortiGate

0) Check L1 - cabling, interfaces, transceivers, etc..

Check the arp table and interface status:

1) get system arp 

2) diagnose ip arp list 

3) diagnose netlink interface list port5 

4) diagnose hardware deviceinfo nic port5 

Run a packet sniffer to verify whether FGT is sending packets and receives them:

5.1 diagnose sniffer packet port5 'host X.X.X.X' 4 0 a - run this command on a separate terminal at the same time with the 5.2 commands (to stop it enter CTRL+C) 

5.2 execute ping-options source Y.Y.Y.Y  # y.y.y.y is IP address of the existing interfaces

execute ping X.X.X.X  # x.x.x.x is default gateway

execute ping 8.8.8.8 

execute ping fortinet.com 

execute traceroute-options source Y.Y.Y.Y

execute traceroute X.X.X.X 

execute traceroute 8.8.8.8