Chapter 11 IPsec VPN for FortiOS 5.0 : L2TP and IPsec (Microsoft VPN) : Troubleshooting : Mac OS X and L2TP
  
Mac OS X and L2TP
FortiOS allows L2TP connections with empty AVP host names and therefore Mac OS X L2TP connections can connect to the FortiGate.
Prior to FortiOS 4.0 MR3, FortiOS refused L2TP connections with empty AVP host names in compliance with RFC 2661 and RFC 3931.