when to use exec / echo external script query

freeradius 1.1.3-1.1 freeradius mysql Redhat Linux Fedora 6 I was looking for information on at what point i would fire an external script. What i wish to do is once a client is authenticated and a framed-ip address allocated to fire an external script to push the IP address into my postfix access table and remove it on disconnect. If anyone has suggestions and can advise on this please chime in. Cory