The design network described in the below URL was proposed to meet the requirements for a national catalog retail business that has approximately 60 retail stores in addition to the direct mail and Internet web business model. The retailer has an existing Cisco VPN 3000 Concentrator that supports remote access software clients, and wants to use that device as an IPSec head end to serve as a crypto peer for dial backup if the primary path over the Internet fails. The application supported is primarily point-of-sale transactions.

http://www.cisco.com/en/US/docs/solutions/Enterprise/WAN_and_MAN/VPNLoad/ch5_Dial_external_docbase_0900e4b180ac09cd_4container_external_docbase_0900e4b180ace543.html#wp1039298

The Cisco VPN 3000 Concentrator supports the Point-to-Point Tunnel Protocol (PPTP) tunneling method for native Windows clients. The concentrator supports 40-bit and 128-bit encryption for a secured reliable connection. The below URL describes how to configure PPTP on a VPN 3000 Concentrator with Cisco Secure ACS for Windows for RADIUS authentication.