Forigate routing

Distance is used select route from different protocols. if there are same routes in the same protocol,  metric then be used select the best route. 

When there are two or more duplicate static routes that have the same distance, FortiGate installs all of them in the routing table. If they also have the same priority, then the routes are known as ECMP static routes, to break ECMP, priority is used to select the best route.

Preferences:

• Distance preference: The lowest value is preferred for the routing table.
• Priority preference: The lowest value is preferred for the routing table.

ECMP

Equal cost multi-path (ECMP) is a mechanism that allows a FortiGate to load-balance routed traffic over multiple gateways. 

ECMP pre-requisites are as follows:

• Routes must have the same destination and costs. In the case of static routes, costs include distance and priority
• Routes are sourced from the same routing protocol. Supported protocols include static routing, OSPF, and BGP

Configure ECMP Load Balancing method

Fortigate-Local (settings) # set v4-ecmp-mode 
source-ip-based         Select next hop based on source IP.
weight-based            Select next hop based on weight.
usage-based             Select next hop based on usage.
source-dest-ip-based    Select next hop based on both source and destination IPs.
 

Verify from CLI

Fortigate-Local # get router info routing-table database 

Codes: ....

Routing table for VRF=0

S    *> 0.0.0.0/0 [10/0] via 10.200.1.254, port1, [1/0]

     *>           [10/0] via 10.200.2.254, port2, [1/0]

C    *> 10.0.1.0/24 is directly connected, port3

C    *> 10.200.1.0/24 is directly connected, port1

C    *> 10.200.2.0/24 is directly connected, port2