Chapter 17 Traffic Shaping for FortiOS 5.0 : Troubleshooting traffic shaping : Interface diagnosis
  
Interface diagnosis
To optimize traffic shaping performance, first ensure that the network interface’s Ethernet statistics are clean of errors, collisions, or buffer overruns. To check the interface, enter the following diagnose command to see the traffic statistics:
diagnose hardware deviceinfo nic <port_name>
See also 
Shaper diagnose commands
Packet loss with statistics on shapers
Packet lost with the debug flow
Session list details with dual traffic shaper
Additional Information