DHCP'ed internet connection not coming up

[2kvolts2005]

This is my setup. Eth0 -->adsl modem ppoe eth1-->cable modem dhcp eth2-->local network 192.168.1.255

the adsl modem comes up just fine and i get a connection to the internet
the cable modem doesnt however come up

i am using shorewall to configure my firewall so that i have two internet connections. is the current adsl connection that does come up preventing the cable modem from coming up?

what do i need to check to get the cable modem to come up?