If
there is any posability of NAT being between
the road-warier and
the VPN server it is very important to set
the value
AssumeUDPEncapsulationContextOnSendRule
to 2 in registry subkey
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPsec
See
http://www.jacco2.dds.nl/networking/freeswan-l2tp.html
and
http://support.microsoft.com/default.aspx/kb/885407/VPN:IPsec:Connect Windows XP to Linux L2TP IPsec VPN server is mentioned on: VPN:IPsec | Windows