DHCP and chnaging the MAC address on a network card

[Pete French]

I have a machine runnign 12-STABLE, which gets its upstream IP address 
from my ISP using DHCP over a cable modem. What I just tried to do was 
to set the MAC address on that interface at boot time, but if I do that 
then it no longer acquires a DHCP address from the upstream.

Is there something going on here I dont understand ?

The interfaces are 'bge' and I have a file called /etc/start_if.bge0 
whcih contains:

	/sbin/ifconfig $1 ether d0:bf:9c:46:a3:7c

and this works (have tried it on bge1 and another machine) in changing 
the MAC address. But if i do that on the DHCP interface then it doesnt 
get an IP address.

Any hints ?

-pete.

PS: reason  am doing this is my ISP will (in future) be able to give me 
a static IP address basd on the MAC address. Which is lovely, but I have 
a spare server I use when this one goes down, so was hoping to have the 
same MAC address if I boot the same drive, to avoid any issues caused by 
the MAc address chnaging if I chnage the hardware. But it doesnt work. :)