Chapter 10 Install and System Administration for FortiOS 5.0 : Monitoring : sFlow : Configuration : Enable sFlow
  
Enable sFlow
config system sflow
set collector-ip <ip_address>
set collector-port <port_number>
end
The default port for sFlow is UDP 6343. To configure in VDOM, use the commands:
config system vdom-sflow
set vdom-sflow enable
set collector-ip <ip_address>
set collector-port <port_number>
end
Configure sFlow agents per interface.
config system interface
edit <interface_name>
set sflow-sampler enable
set sample-rate <every_n_packets>
set sample-direction [tx | rx | both]
set polling-interval <seconds>
   end
See Also
sFlow
Configuration
Monitoring