Cisco IPsec VPN Active Directory Authentication

active-directoryciscovpn

I've been trying to get our Cisco ISR 2811 to authenticate the VPN clients towards an Active Directory server (so that the Cisco VPN client asks the user for username/password).

Googling around has only given me instructions on how to do it with a PIX/ASA, I would like to do the same on the router.

Thanks,
Mattias