The FortiGate unit has a number of different options for configuring interface address options
Network > Interfaces > Select interface and click Edit
Manual – Allow for entering in a Static IP address with subnet mask
DHCP – retrieves IP address from a DHCP server, For example set on WAN interface if ISP issues IP address via DHCP
Retrieve default gateway: This get the default gateway from DHCP server and enters it into the routing table.
Distance: The distance metric added to the default route.
Override internal DNS: This replaces the information in the internal DNS field (see here for internal DNS) with values from DHCP server.
PPPoE – For connections that need a PPPoE username/password for authentication
Username: PPPoE account username
Password: PPPoE account password
Unnumbered IP: Allow to specify the IP address of the interface
Initial Disc Timeout: The time to wait for retrying the PPPoE discovery after a timeout
Initial PADT Timeout: Sutdown the PPPoE session if idle for the amount in the timeout. This must be supported by ISP to work
Retrieve default gateway: This get the default gateway from DHCP server and enters it into the routing table.
Distance: The distance metric added to the default route.
Override internal DNS: This replaces the information in the internal DNS field (see here for internal DNS) with values from DHCP server.