With RADIUS authentication enabled, VPN clients authenticate to Active Directory, which provides centralized remote access authentication, authorization, accounting, and auditing. This Link may help,
RADIUS Configuration and Issues
RADIUS authentication ... Remote Authentication Dial-In User Service (RADIUS) is an industry standard protocol described in ...
http://www.howtonetworking.com/articles/radius.htm
Bob Lin, MS-MVP, MCSE & CNE
Networking, Internet, Routing, VPN Troubleshooting on
http://www.ChicagoTech.net
How to Setup Windows, Network, VPN & Remote Access on
http://www.HowToNetworking.com
What are the advantages of using raduis authentication to authenticate
pptp or lt2p/ipsec clients. Thanks