iptables with multiple links

Hi,
We have configured one IPTABLES linux with NAT. Default external IP is
through 2Mbps leased line. Behaind our Iptables, one DHCP server is
running for the labs.
Now we have added one back up ISDN connection which gets activated
automatically nce the leased line goes down. The router makes automatic
connection but with ISDN connection new static IP is given for the back
up connection.
Now my question is what rules must be set in the default Iptables set
so that the system works automatically with leased line and also when
leased line is down(with ISDN back up conection)?
Any help shall be appreciated.
Thanks
-Gaurms