Configuring RADIUS on a cisco is a three step process:
radius-server host "ipaddress"
radius-server key "secret"
AAA authentication login use-radius radius
AAA authentication PPP use-radius if-needed radius
! exec authorization does per-user services after ascii login
AAA authorization exec radius if-authenticated
! network authorization permits per-user access lists for PPP/etc
AAA authorization network radius if-authenticated
! Accounting is optional
AAA accounting exec start-stop radius
AAA accounting network start-stop radius
line 1 16
login authentication use-radius
autoselect PPP
autoselect during-login
!
interface group-async 1
ppp authentication chap use-radius