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

Tom Wolfe twolfe at sawback.com
Tue May 6 20:53:56 UTC 2008

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?

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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