Technical Note: How to configure L2TP using interface/route based IPsec VPN:

http://kb.fortinet.com/kb/microsites/search.do?cmd=displayKC&docType=kc&externalId=FD41147&sliceId=1&docTypeID=DT_KCARTICLE_1_1&dialogID=26850343&stateId=1%200%2026848963