Chapter 1 What’s New for FortiOS 5.2.1 : Advanced Routing : Source and Destination IP-based Mode for ECMP
  
Source and Destination IP-based Mode for ECMP
A new mode has been added that allows Equal-cost multi-path routing (ECMP) to select next hop based on both source and destination IPs. This can be configured as a global setting (if virtual-wan-link is disabled) or for a virtual WAN link.
Syntax
config system {settings | virtual-wan-link}
set v4-ecmp-mode source-dest-ip-based
end