Chapter 11 IPsec VPN for FortiOS 5.0 : IPsec VPN Overview : Planning your VPN
  
Planning your VPN
It is a good idea to plan the VPN configuration ahead of time. This will save time later and be help you configure your VPN correctly.
All VPN configurations comprise a number of required and optional parameters. Before you begin, you need to determine:
where does the IP traffic originate and where does it need to be delivered
which hosts, servers, or networks to include in the VPN
which VPN devices to include in the configuration
through which interfaces the VPN devices communicate
through which interfaces do private networks access the VPN gateways
Once you have this information, you can select a VPN topology that meets the requirements of your situation.