Recently, Microsoft and Cisco agreed to merge their respective protocols into a single, standard protocol called Layer Two Tunneling Protocol(L2TP).

Cisco's Layer Two Forwarding Page
Includes links to news releases, technical tips, and articles.

Solutions for Virtual Private Dialup Networks
White paper from Cisco Systems that describes the L2F protocol.

Unofficial L2TP Web Page
Contains links to IETF documents and other Web pages related to L2TP.