Chapter 11 IPsec VPN for FortiOS 5.0 : L2TP and IPsec (Microsoft VPN) : Troubleshooting
  
Troubleshooting
This section describes some checks and tools you can use to resolve issues with L2TP-over-IPsec VPNs.
This section includes:
Quick checks
Mac OS X and L2TP
Setting up logging
Using the FortiGate unit debug commands