Fortinet FortiGate 100 Installation & Configuration Manual page 56

Firewall policy example
56
Restricting access to a single Internet connection
In some cases you might want to limit some traffic to only being able to use one
Internet connection. For example, in the topology shown in
organization might want its mail server to only be able to connect to the SMTP mail
server of ISP1. To do this, you add a single Int->Ext firewall policy for SMTP
connections. Because redundant policies have not been added, SMTP traffic from the
Internet network is always connected to ISP1. If the connection to ISP1 fails the SMTP
connection is not available.
NAT/Route mode installation
Figure 8 on page 50 the
Fortinet Inc.