cisco vpn & radius

hi i would like to configure my cisco 2620 router as VPN concentrator with a 12.3 IOS and i use a radius server (freeradius) to authenticate the users of the VPN but when the radius get the ip config to the router then the router don't relay these informations to the VPN client and it assigns an ip in its pool. i don't know how must i config the router in order that it uses the radius ip config to configure the VPN client... if i delete the pool from the router then i cannot connect on the VPN router and i didn't find any configuration to specify the router to use the ip config from the radius... i hope i'm clear !!