Draytek Vigor2920 Series User Manual page 314

Available settings are explained as follows:
Item
Allowed Dial-In
Type
Vigor2920 Series User's Guide
Description
Determine the dial-in connection with different types.
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 with IPSec Policy - 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.
Specify Remote VPN Gateway - You can specify the IP
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