The strangest cable modem issue

I thought I’d ask NANOG this, since somebody may be from a large cable operator and may know. I am a Comcast customer and don’t want to call this in through tech support, as I’ve tried that before without any success.

I use my cable modem normally – to surf the web, to play a game or two, and to listen to stream casts. I have a buddy who often needs tech support, so I use RAdmin ( to login to his computer and go through stuff with him. I use large TCP packets, small UDP packets – ICMP, ESP – large and small.

RAdmin can transfer files from desktop to desktop. Ethereal reveals this is a standard TCP based protocol used on the RAdmin port tcp/4899. If I want him to have a file, I push it that way.

Last night – I decided to start IIS FTP server, and have him use his SmartFTP ( client to pull several larger files from me. I poked a hole in the Linksys router’s port forwarding for FTP and off we went. He grabs the first file, and it’s going along at roughly the correct uplink speed for Cable (20 KB/s or so) and all is fine. Then, my cable modem reboots. Stops dead. All the lights go off and start blinking, a full reload. It grabs the same IP address via DHCP and he restarts. Again, about 15 seconds into the download, it crashes the cable modem. We do this about 5 times to really make sure it’s just FTP doing this.

I have a DOCSIS 1.1 modem from RCA (,2588,PI700502-CI700094,00.html?)

I have a Linksys BEFSR81 ( with the latest firmware

FTP is the only protocol that does this. Using RAdmin file transfer, I push files to him over tcp/4899 and it doesn’t happen.

The cable modem is somewhat old, perhaps it needs to be flashed with a newer image, but I am not sure what could cause it.


I think your problem was as a result of what happened to Comcast yesterday

Was it only happening yesterday?



NANOG is not tech support. Comp.dcom.modems.cable is more appropriate. point 2.

Get rid of the "?1c" at the end of the URL or it will take you a
registration page as it did for me.

Also, if there was a network outage, the modem wouldn't reboot itself. I
have this same problem when I'm nearly maxing out my WAN upload bandwidth
on my home DSL line but it only happens on my cheap D-Link router. I think
it a CPU usage issue for the device because when your maxing out your
upload, the network will be really bogged down so if the modem keeps
getting more and more traffic it would become too much for it's processor
to handle so it reboots.

The good reason why it does that which I can come up with is that during a
DoS attack, the same network conditions are experienced the the modems CPU
would be overwhelmed, so instead of it just hanging there the device is
rebooted so it would function again.

If it were possible for you to connect into your modem and see what's
happening with the CPU then perhaps that will give a few clues but at the
moment, it's anyone's guess.