Draytek 5500 Series manual Allowed Dial-In Type, IPSec Tunnel

Allowed Dial-In Type	Determine the dial-in connection with different types.
ISDN	Allow the remote ISDN LAN-to-LAN connection. You
	should set the User Name and Password of remote dial-in user
	below. This feature is useful for i model only. In addition, you
	can further set up Callback function below.
PPTP	Allow the remote dial-in user to make a PPTP VPN
	connection through the Internet. You should set the User
	Name and Password of remote dial-in user below.
IPSec Tunnel	Allow the remote dial-in user to trigger an IPSec VPN
	connection through Internet.
L2TP	Allow the remote dial-in user to make a L2TP VPN
	connection through the Internet. You can select to use L2TP
	alone or with IPSec. Select from below:
	None- Do not apply the IPSec policy. Accordingly, the VPN
	connection employed the L2TP without IPSec policy can be
	viewed as one pure L2TP connection.
	Nice to Have- Apply the IPSec policy first, if it is applicable
	during negotiation. Otherwise, the dial-in VPN connection
	becomes one pure L2TP connection.
	Must- Specify the IPSec policy to be definitely applied on the
	L2TP connection.
116	VigorPro5500 Series User’s Guide