Creating the GRETAP interface
In the Network > Interfaces module, click on Add and select Add a interface. Enter the following mandatory fields:
Name: assign a name to the GRETAP interface (gretap in the example).
Bridge: select an existing bridge on the firewall. This may be a bridge created by the default configuration or a bridge created specifically for this purpose.
Bridges cannot be created in the GRETAP interface creation wizard.
It is possible to not assign any bridge to the interface by selecting the option Create an inactive GRETAP interface. The interface can then be enabled later by moving it to a bridge.
GRETAP tunnel configuration
Tunnel source: select the physical interface through which GRE traffic will travel on the firewall. In the example given, this is the Firewall_out interface.
Tunnel destination: select an object bearing the public IP address of the remote firewall (Remote_Firewall in the example).
Click on Finish then Apply to confirm the creation of the GRETAP interface.