Fortinet FortiGate 50A Installation And Configuration Manual page 214

Configuring L2TP
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4 Go to the Options tab and select IP security properties.
5 Make sure that Do not use IPSEC is selected.
6 Select OK and close the connection properties window.
Note: The default Windows XP L2TP traffic policy does not allow L2TP traffic without IPSec
encryption. You can disable default behavior by editing the Windows XP Registry as described
in the following steps. See the Microsoft documentation for editing the Windows Registry.
7 Use the registry editor (regedit) to locate the following key in the registry:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\
Parameters
8 Add the following registry value to this key:
Value Name: ProhibitIpSec
Data Type: REG_DWORD
Value: 1
9 Save the changes and restart the computer for the changes to take effect.
You must add the ProhibitIpSec registry value to each Windows XP-based
endpoint computer of an L2TP or IPSec connection to prevent the automatic filter for
L2TP and IPSec traffic from being created. When the ProhibitIpSec registry value
is set to 1, your Windows XP-based computer does not create the automatic filter that
uses CA authentication. Instead, it checks for a local or active directory IPSec policy.
To connect to the L2TP VPN
1 Connect to your ISP.
2 Start the VPN connection that you configured in the previous procedure.
3 Enter your L2TP VPN User Name and Password.
4 Select Connect.
5 In the connect window, enter the User Name and Password that you use to connect to
your dialup network connection.
This user name and password is not the same as your VPN user name and password.
PPTP and L2TP VPN
Fortinet Inc.