Virtual Private Dialup Network (VPDN)

A VPDN is a network that extends remote access to a private network using a shared infrastructure. VPDNs use Layer 2 tunnel technologies (L2F, L2TP, and PPTP) to extend the Layer 2 and higher parts of the network connection from a remote user across an ISP network to a private network. VPDNs are a cost effective method of establishing a long distance, point-to-point connection between remote dial users and a private network.

Reading:

http://www.cisco.com/en/US/tech/tk801/tk703/tsd_technology_support_protocol_home.html

Cisco VPDN Configuration Examples and TechNotes