Though I know you dealt with this awhile ago, after the last couple of days I feel your pain. I've burned through at least 15 solid hours trying to get my Linksys router to generate a DHCP request that Comcast's servers don't simply ignore. And the annoying thing is that any computer I plug directly into the cable modem has no problem getting a Comcast IP address (and I've tried several).

The most ironic twist is that the router (a WRT54G v.3.1) worked fine with the default Linksys firmware. But after a day or two (when it became clear how much of a piece of junk the default firmware is), I flashed it to OpenWRT, then later Tomato once I couldn't get OpenWRT to get an IP address. Under either, I've yet to be successful in getting the router to get a valid WAN IP address from Comcast.

I wasted a ton of hours researching the problem on the net, so I came across a whole pile of problems that people have apparently tried and succeeded with at various times. Though they've all failed for me, I'll compile them here for anyone else stumbling by.

1. All sorts of combinations of rebooting modems, routers, replugging cables, waiting minutes/hours while unplugged for things to reset, etc.
2. Clone the MAC address of the router to that of a host computer that CAN connect directly through the cable modem.
3. Minimizing the size of DHCP packets through the Tomato firmware -- some people thought that Comcast was dropping DHCP packets over a certain size. Again, doing this did nothing for me.
4. Messing with various other obscure router settings (firewall details, frame sizes, etc).
5. Enabling/disable STP on the router (spanning tree protocol). Apparently STP causes DHCP to fail on Comcast's network, but it was already disabled on Tomato. I tried enabling this then disabling again to no effect.

All I've been able to get is a constant "renewing DHCP lease" status on the firmware. The major thing that implicates Comcast's network is that my cable modem (some Ambit model supplied by Comcast) sees the DHCP requests from my router and forwards them, then logs errors when no response is ever received from Comcast's servers. So the next time I go look at my modem's logs, I see a whole list of "DHCP FAILED - Discover sent, no offer received" errors. So clearly, my router is properly communicating with the modem, and Comcast's servers simply choose to ignore the DHCP requests from my router for some reason.

At this point, I gave up and braved the trial by fire (Comcast tech support). The first guy I got on the phone went through the usual battery of "unplug/replug your stuff", then the usual "your modem looks fine to me, everything checks out", then got to what would become his mantra for the next half hour, "It's your router's fault. Change your firmware and/or your router. It's not our problem."

Of course, every time he fed me this argument, he couldn't give me a good reason why, if it was my router's fault, the DHCP requests were still getting sent out to Comcast's servers and then causing errors to get logged on my modem (NOT my router) after Comcast's DHCP servers never replied. Either way, my DHCP requests were reaching Comcast's servers -- the missing link was that Comcast was selectively ignoring my requests. He kept insisting that since my computer worked when plugged in, 1) it was clearly my router's fault, and 2) it wasn't his problem. I pointed out several times that DHCP working *some* of the time didn't mean that Comcast's servers weren't at fault -- it meant that Comcast's servers silently ignore certain types of DHCP requests either because of problems on the server or because of completely undocumented configuration settings. My DHCP requests were clearly OK, seeing as Comcast's own modem was complaining when responses to them were never received.

It took a solid 45 minutes of arguing with the level 1 guy (during which he took several breaks to talk to some of the other tech support people there, who basically gave me the same arguments) before he finally escalated me to level 2. After another 5 minutes explaining the problem to the level 2 woman I talked to, she decided to take another approach to getting rid of me: telling me to call someone else, namely the router manufacturer (Linksys). I pointed out several times that this would clearly be fruitless, since the Linksys people would just tell me (and rightfully so) that since I wasn't even using their firmware, they couldn't help me. Unfortunately, she didn't seem to get that this was clearly a software problem, not a hardware problem.

At this point, she was pushing harder to get me off the phone, with lots of "if your modem connects directly to your computer okay, it's not our problem", and refusals to acknowledge that something might be off with Comcast's network. She also refused to transfer me to anyone else who might actually know some relevant information. At this point having decided that the likelihood of my problem getting solved by tech support was pretty much nil, I started trying to get her to send a message about the problem to somewhat at Comcast with the capability to actually do something about it (e.g., a network engineer or the like). She said she'd send a message and open a support ticket, but that it might take awhile -- I think that can safely be interpreted as "we'll make a low-priority ticket that nobody will actually pursue or care about since it's not specific or easy enough to solve in 10 minutes". I suppose there's always that 2% chance that it'll get someone in Comcast who actually knows what they're talking about to do something about it. But I'm not counting on it.

In the meantime, my router is still as useless as it was 3 days ago. I'm sure I could just go buy another router and it'd work. But after sinking so much time into a project, accepting defeat from Comcast just sucks.

But at least one of the people I talked to in the Comcast phone maze tried to get me to sign up for Digital Voice at a Special Promotional Rate. Thanks, Comcast.

RE: Comcast's DHCP is broken