You should have some scripts located in /etc/ppp/ by the name of ip-up.local and ip-down.local. (there should be several in there) You can edit these to do what you want them to. Ip-up.local is called from ip-up when ppp is assigned an ip address. Ip-down is just the opposite.
To be fair, I use Mandrake so Red Hat may have slightly different names for the scripts. Look in the directory /etc/ppp. It should be simple enough to figure out.
Hope this helps,