В чем отличие?
Cisco writes

Q. What are the differences between L2F, PPTP, and L2TP?

A. The differences from a user's perspective are general availability more than technical details of the protocols. All three protocols tunnel PPP, thus they are similar. They are all Layer 2 tunneling protocols that support Access VPN solutions. L2TP represents the best of both L2F and PPTP combined.

Это не очень проясняет суть вопроса