Well, the Comcast guy showed up today and dropped off the modem self-installation kit. About two or three hours later and it still wasn't working quite right.
Here is my setup. The modem is plugged into eth0 on a Linux box, and eth1 runs to a switch. Traffic routed out eth0 is masqueraded (also known as NATed). If the modem and Linux box are turned on then everything works fine. But if I bring eth0 down and back up, then some odd behavior begins. All traffic that originates on the Linux box behaves normally -- I can use elinks to browse the web and irssi to chat on IRC without any problems. But any traffic that is masqueraded, meaning that it comes from another computer on my network, does not behave normally. The connection establishes and works for a split second and then is silent.
I'd suspect a routing problem on the Linux box, but tcpdump there confirms everything is working as it should. However, ifconfig reports RX errors on eth0. This makes no sense -- traffic originating from the LAN side of the router box triggers receive errors on eth0. Unless I reboot the router and the modem. I have not observed this behavior with any other ISP or uplink switch.
Anyone have any theories?