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/