Chapter 10 Install and System Administration for FortiOS 5.0 : PPTP and L2TP : Configuring L2TP VPNs : Network topology
  
Network topology
The remote client connects to an ISP that determines whether the client requires an L2TP connection to the FortiGate unit. If an L2TP connection is required, the connection request is forwarded to the FortiGate unit directly.
See Also
Configuring L2TP VPNs
L2TP infrastructure requirements
PPTP and L2TP
Figure 245: Example L2TP configuration