IPv4 and IPv6 ECMP load balancing

You can use the following command to configure the IPv4 ECMP load balancing method for a VDOM:

config system settings

set v4-ecmp-mode {source-ip-based | weight-based | source-dest-ip-based | usage-based}

end

With VDOM-based session tables enabled, the FortiGate-6000 supports all IPv4 ECMP load balancing methods except usage-based. If you select usage-based, all IPv4 traffic uses the first IPv4 ECMP route instead of being load balanced among all Ipv4 ECMP routes. All other IPv4 ECMP load balancing methods are supported.

See this link for information about how to support IPv6 ECMP load balancing: Technical Tip: ECMP – Load balancing algorithms for IPv4 and IPv6.