Chapter 4 FortiOS Carrier : Carrier web-based manager settings : GTP Profile : Anti-Overbilling options
  
Anti-Overbilling options
You can configure the FortiOS Carrier firewall to prevent overbilling subscribers for traffic over the. To enable anti-overbilling, you must configure both the Gn/Gp firewall and the Gi firewall.
Expand Anti-Overbilling in the GTP profile to reveal these settings.
 
Anti-Overbilling section of the New GTP Profile page
Gi Firewall IP Address
The IP address of the unit’s interface configured as a Gi gateway.
Port
The SG security port number. The default port number is port 21123. Change this number if your system uses a different SG port.
Interface
Select the unit interface configured as a Gi gateway.
Security Context ID
Enter the security context ID. This ID must match the ID entered on the server Gi firewall. The default security context ID is 696.