Chapter 11 IPsec VPN for FortiOS 5.0 : Dynamic DNS configuration : Dynamic DNS over VPN concepts : Dynamic DNS (DDNS) : Routing
  
Routing
When an interface has some form of changing IP address (DDNS, PPPoE, or DHCP assigned address), routing needs special attention. The standard static route cannot handle the changing IP address. The solution is to use the dynamic-gateway command in the CLI. Say for example you already have four static routes, and you have a PPPoE connection over the wan2 interface and you want to use that as your default route.
The route is configured on the dynamic address VPN peer trying to access the static address FortiGate unit.
To configure dynamic gateway routing - CLI
config router static
edit 5
set dst 0.0.0.0 0.0.0.0
set dynamic-gateway enable
set device wan2
next
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