Modify the routing behavior of an SSG5-Serial firewall.

Folks,

Presently we have configured VPN on a Juniper SSG5-Serial firewall. The default route this firewall gets is over it's Untrust Interface because the Untrust Interface receives an DHCP IP address. This default route get added as a Connected route and is of course preferred.

Our goal is to ensure that the default route is a static route which is added on a tunnel Interface. How can we achieve that? If we add a default static route with preference as 20 this does not get preferred.

Thanks!!

N.