VPN authentication using LDAP

With more and more users requesting L2TP/IPsec VPNs, I find it impractical to keep track of which users are still active in our organization. I would like to let LDAP handle the VPN authentication for these users.  Is that possible?  I am able to connect to the LDAP server and authenticate as a firewall user, but I can't seem to figure out how get VPN connections to accept those ldap users.  

Any guidance or experience is appreciated.

Thanks, Bill.