Changing the settings of the traffic’s source physical interface and moving it to the bridge

On each firewall that is part of the GRETAP tunnel:

1. In the Configuration > Network > Interfaces module, double-click on the source physical interface of the traffic that needs to go through the tunnel.
   In the example shown, this will be the in interface.
   
2. In General settings > This interface is field, select External (public).
3. In Address range > Address range field, select Address range inherited from the bridge.
4. In Address range > Bridge field, select the bridge created earlier (new_bridge1 in the example).
   The interface is automatically moved to the bridge new_bridge1.
5. Go to the Advanced properties tab.
6. In Routing by interface, select Keep initial routing.
   A Keep VLAN IDs check box appears. Select it.
7. Click on Apply to confirm the creation of the GRETAP interface.