DHCP troubles with Draytek Vigor2104 and cablemodem Thomson TCM315

Hello everybody,

I have an internet connection through a Draytek Vigor 2104 router and
a Cablemodem. It was necessary to install a SB5100 cablemodem because
it was impossible with a TCM315. When the router ask for a IP address,
all DHCP process seem to be right, but the router is asking for ip
addres continually, several times per second. Router never becomes
online beacuse of this.

Does anybody know the reasons for this behavior and how could this
problem be fixed? (without chaging cablemodem, of corurse...)

Thanks a lot,
Fernando