As it was explained to me, if you put a router on our cable system and don't notify the provider that it's there the modem won't be able to start up again. Our cable system has all digital modems now and this may be why. Perhaps on the older analog systems this might not have been all that critical from a performance standpoint. I know someone I can ask about all of this and probably get more information than Linus Torfalds could handle though.
