[K12OSN] Excessive collisions cripple network -- Suggestions for solutions?

Huck dhuckaby at paasda.org
Tue May 6 21:01:04 UTC 2008

one that wasn't bought at bestbuy/target/circuit city...
not a SOHO switch...

Tom Wolfe wrote:
> Thanks for the tips, insights and suggestions everyone.
> I'm going to try to as many of these issues as possible, starting with 
> cleaning up the wiring. You're right Chuck, I'm certain that most of 
> this has to do with the rats nests of Cat 5 cables behind the rows of 
> computers leading to the cheap switches.
> What is an "IT-owned" switch btw? Do you mean a switch that wasn't 
> bought & installed by a teacher, i.e. one that I'm aware of?
> Regards,
> Tom Wolfe
> On Mon, 5 May 2008, Chuck Kollars wrote:
>>> ... Every couple of months it seems that
>>> accidentally ?someone? plugs both ends of an
>>> ethernet cable into the same switch. This ends up
>>> sending out collisions ...
>> It's actually a broadcast storm, not a jam. Something
>> like wireshark will show the whole network is
>> saturated with repeats of a "legitimate" packet, and
>> that packet is addressed to broadcast. The "jam" light
>> is not actually on.
>> And the enabling culprit is that "auto-crossover"
>> function on many of the little hubs. It starts with a
>> legitimate broadcast packet going out all the ports.
>> The packet traverses the loop and goes right back into
>> the mini-hub, which understands it because of the
>> auto-crossover. What's the right thing to do when a
>> broadcast packet is received? Send it right back out
>> all the other ports again of course  ...repeat as
>> needed.
>> Although there are "proper" solutions involving either
>> high-quality switches or a hierarchical network
>> architecture, either of the low-tech solutions below
>> will work too:
>> low-tech alternative 1) Put an IT-owned switch in each
>> classroom and ban mini-hubs (at least "most" of them).
>> Make sure the IT-owned switches are "stupid" enough to
>> _not_ have the auto-crossover. If you're going to do
>> this, make sure the administration will back up your
>> banning of non-IT-installed minihubs.
>> low-tech alternative 2) Every few weeks walk through
>> the building and "take" all the unnecessary or
>> dangling CAT5 jumper cables. Make it so teachers have
>> to come see you to get a CAT5 cable. If there's no
>> dangling end, a "helpful" kid can't plug it into the
>> hub. Surprisingly, this silly solution works pretty
>> well for us; we have a single network covering over a
>> thousand students and we only have one of these "loop"
>> indicents about once every three years.
>> good luck!
>> -Chuck Kollars
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