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Finding the Truth Behind Cable Modem Traffic Bursts?

Posted by Cliff on from the the-game-is-afoot! dept.

Techi asks: "I help to support a small cable modem network in Kansas, and we keep having recurring problems with all the modems in a node bursting extreme amounts of traffic for a period of about 30 seconds. At the end of this 30 second period, the upstream port that the node in question is connected to dies under the pressure. We have recently implemented a fix to keep people from uncapping modems in the form of a config file update from our DHCP system. I know we could have done it differently, but it wasn't my decision. Does anyone have any idea what could be causing 70 or so modems at a time to suddenly erupt with outgoing traffic nonstop until the upstream dies?"

3 of 31 comments (clear)

  1. Try monitoring by MarkusQ · · Score: 4, Interesting
    Why don't you try monitoring? Leave something tcpdump-ish on the line, saving the last however many packets until something dies. Then dust for finger prints.

    I've seen something like this happen and suspect either dumb/misconfigured DHCP clients, an election process run amok, or some sort of ICMP flurry. No proof either way, since in this case I'm just a user & I just wait it out.

    -- MarkusQ

    P.S. I have noticed an interesting patterm to the timing though. You might try looking at the times / dates of past events to see if that suggests anything (and it can often suggest a lot).

  2. You need to capture the data by Yarn · · Score: 4, Interesting

    To me it sounds like what I've heard called an 'ACK Storm'.

    It seems to occur when a switch somewhere gets it's MAC table corrupted somehow and starts squirting rubbish onto the network.

    I accidentally caused one of these at my uni, by changing the MAC address of my netcard, it brought down the whole network for hours, the switch was continuously broadcasting the last packet it saw.

    They never found it was me though ;)

    --
    -Yarn - Rio Karma: Excellent
  3. Sounds like a broadcast storm by schon · · Score: 4, Informative

    Sounds like a broadcast storm to me..

    One (misconfigured) machine broadcasts data (say, NMB update) with a source address of the broadcast address - everybody on the segment replies, (which causes everybody on the segment, including the misconfigured one to reply again, ad infinitum) - result: segment meltdown.

    As someone else pointed out, a traffic monitor would be your best bet - you don't need to capture all of it, just the first part, to see what's starting it up - then you can decide what to do.