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

Chuck Kollars ckollars9 at yahoo.com
Mon May 5 19:34:13 UTC 2008

> ... 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

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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