Recently I needed to make sure select traffic would flow over a certain ISP link. Unfortunately that link had a dynamic address, which meant the address and gateway of that route could change anytime. Also I wanted to have my primary ISP failover to this link if needed.
To accomplish these things I needed to have both default routes in my routing table at the same time. This means that they both have the same distance, but different priorities. One way to accomplish this is to configure a static default route, and just change the priority of the link , but how can you do this when you do not know the gateway?
You can create a dynamic-gateway static route in the Fortigate.
Through CLI you can create a dynamic gateway route using the above syntax. Remember, the higher the priority the less preferable the route.
You can also create basically the same thing under the interface of the WAN link by using the distance, and priority interface commands listed below:
So now if we check our route monitor:
We have both default routes, and can successfully use a policy based route to push the needed traffic out.
Very useful information when you have DHCP enabled on the WAN interface and the ISP is changing their addressing scheme once in a while.