Chapter 2 Advanced Routing for FortiOS 5.0 : Advanced Static Routing : Routing concepts : Route priority
  
Route priority
After the FortiGate unit selects static routes for the forwarding table based on their administrative distances, the priority field of those routes determines routing preference. Priority is a Fortinet value that may or may not be present in other brands of routers.
You can configure the priority field through the CLI or the web-based manager. Priority values can range from 0 to 4 294 967 295. The route with the lowest value in the priority field is considered the best route. It is also the primary route.
To change the priority of a route - web-based manager
1. Go to Router > Static > Static Routes.
2. Select the route entry, and select Edit.
3. Select Advanced.
4. Enter the Priority value.
5. Select OK.
To change the priority of a route - CLI
The following command changes the priority to 5 for a route to the address 10.10.10.1 on the port1 interface.
config router static
edit 1
set device port1
set gateway 10.10.10.10
set dst 10.10.10.1
set priority 5
end
If there are other routes set to priority 10, the route set to priority 5 will be preferred. If there are routes set to priorities less than 5, those other routes will be preferred instead.
In summary, because you can use the CLI to specify which sequence numbers or priority field settings to use when defining static routes, you can prioritize routes to the same destination according to their priority field settings. For a static route to be the preferred route, you must create the route using the config router static CLI command and specify a low priority for the route. If two routes have the same administrative distance and the same priority, then they are equal cost multipath (ECMP) routes.
Since this means there is more than one route to the same destination, it can be confusing which route or routes to install and use. However, if you have enabled load balancing with ECMP routes, then different sessions will resolve this problem by using different routes to the same address.