PPPoE address change

[ming fu]

Hi,

I was using Bell sympatico pppoe. The PPP tunnel address changes every 
few hours. Is there a way to let ppp to call some program so that 
servers and ipfw rules can be changed to adjust to the new IP.

I know I can run a cron job checking the routing state every few 
seconds, but I want my ipfw rule to change write way when the tunnel 
address changes.

Thanks, 
Ming