http://en.wikipedia.org/wiki/L2TP

http://www.cisco.com/warp/public/cc/pd/iosw/tech/l2pro_tc.htm

http://www.answers.com/topic/layer-2-tunneling-protocol

http://www.webopedia.com/TERM/L/L2TP.html

http://www.cisco.com/en/US/docs/ios/12_0t/12_0t1/feature/guide/l2tpT.html

http://support.microsoft.com/kb/240262

http://www.mikrotik.com/testdocs/ros/3.0/vpn/l2tp.php

http://support.microsoft.com/kb/259335

http://www.microsoft.com/windows/windows2000/en/advanced/sag_RASS_scen_l2tp_rc.htm

http://technet.microsoft.com/en-us/library/bb742553.aspx

http://www.ietf.org/rfc/rfc2661.txt

http://www.alliedtelesyn.co.nz/documentation/relnotes/7.6.1/rn107615.html

http://www.microsoft.com/windows/windows2000/en/advanced/sag_RRAS_scen_l2tp_bo.htm

http://www.jacco2.dds.nl/networking/openswan-l2tp.html

http://www.networksorcery.com/enp/protocol/l2tp.htm

http://www.freebase.com/view/en/l2tp

http://www.linktionary.com/l/l2tp.html

http://www.experts-exchange.com/Networking/Broadband/VPN/Q_21172879.html

http://www.javvin.com/protocolL2TP.html

http://technet.microsoft.com/en-us/library/cc977622.aspx