A puzzling router problem

Friday, November 14th, 2008 at 7:25 pm

This morning when I was woken up by the thunderstorms I noticed that there must have been a brief power outage as my stereo had reset. So I checked Preston, my linux server, and found while that it had been interrupted, it had restarted without any problems.

But I didn’t have internet.

The router was still up and the cable modem had all the normal lights on so it looked ok. I just could access any external sites. I case there was a problem with the cable network because of the weather that just wasn’t being shown I power cycled both of them.

When I checked it again the cable modem had reconnected and the router was running, but the router hadn’t been assigned an IP address. That’s not right, but I had to go to work and I hoped that it was just a problem with the DHCP server that would be fixed by the time I got home.

It wasn’t, and another reboot didn’t fix it.

Against the possibility that something was wrong with the router I connected my desktop directly to the cable modem. This worked which indicated there was nothing wrong with the cable network. So I tried the router again, it didn’t work.

In case the problem was just with the DHCP client in the router I tried setting the connection details manually. This still didn’t work.

The cable modem only accepts one client MAC address at a time, so each time I changed between my desktop and the router I needed to power cycle the cable modem. To save time I had the modem clone the MAC address of my desktop. This made it work. Yes, this doesn’t make sense.

After testing all the combinations again (including resetting the cable modem to defaults) I foudn that when using the router’s own MAC address no traffic gets through. However if I have it clone another address, even one I make up, it all works as before.

This setup has worked fine for a couple of years, what changed this morning?

I’m puzzled.

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