I have replaced and old 2003 DC with a Server 2008 box a few years back and moved all the FMSO roles over to the 2008 box. Now we have noticed if we reboot the 2003 DC everyone losses internet.
We don't use DHCP and all of the computers have been setup with static IPs.
The nic's of the computers are configured so the Default GW is the main switch's IP, and the DNS 1 is the 2008 box and DNS 2 is the old 2003 box.
I did a route print on both servers, and did notice that the metrics on the 2008 box were much much higher than the 2003 box, like 306 an 276 for the route metrics.