Define a new AAA (Authentication, Authorization, Accounting) model

!
aaa authentication login default group radius local
aaa authentication login login-local line local enable
aaa authentication login noauth line none
aaa authentication login netlogin line none
aaa authentication ppp default group radius
aaa authorization network default group radius 
aaa accounting network default start-stop group radius
aaa session-id common
!

Disable authentication for the serial console

!
line con 0
 login authentication noauth
!

Enable the VPDN (Virtual Private Data Network) Server

!
vpdn enable
!
vpdn-group pptp-backup
! Default PPTP VPDN group
 accept-dialin
  protocol pptp
  virtual-template 1
 local name asyd
!

The virtual interface which is use as template when a user connect

!
interface Virtual-Template1
 ip unnumbered Ethernet1
 ppp encrypt mppe 40 required stateful
 ppp authentication ms-chap
 ppp multilink
!