Usually when setting up a new firewall, the interface speeds, specifically the WAN interface speed is set to “auto” and will attempt to negotiate the speed of the connection. Most of the time this is OK and you don’t have to adjust this setting and the connection will work fine. Recently, I ran into an issue where the Internet speeds were very slow, and users could not get to a lot of sites necessary to complete their work.
After some troubleshooting, I found that the ISP needed the connection statically set at 10/Full to work with the service they were delivering. After poking around on the FortiGate I saw that the WAN1 interface setting was set to “auto” by running this command:
#show system interface wan1
After confirming the speed that needed to be set on this interface to work with the ISP I used the commands:
#config system interface
#set speed 10full
This immediately resolved the slow Internet speed issues. Next time you have symptoms similar to what is described above, check the interface speed.