Chapter 5 FortiOS Carrier : Troubleshooting : GTP packets are not moving along your network : Perform a sniffer trace
  
Perform a sniffer trace
When troubleshooting network traffic, it helps to look inside the headers of packets to determine if they are traveling along the route you expect. Packet sniffing can also be called a network tap, packet capture, or logic analyzing.
 
If your Carrier-enabled FortiGate unit has NP interfaces that are offloading traffic, this will change the sniffer trace. Before performing a trace on any NP interfaces, disable offloading on those interfaces.