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

"Terrell Prudé Jr." microman at cmosnetworks.com
Sun May 4 21:01:29 UTC 2008

I guess it depends on what you're looking for.  If you're looking for
just *one* switch, specifically for a K12LTSP lab, then you can go with
pretty much any of the managed switches below.

On the other hand, it sounds from your initial email like your site has
a "cobbled-together" network infrastructure consisting of CompUSA-grade
mini-switch devices.  If I were you, I'd be looking at a wholesale
upgrade of the wiring closets.  That's what we did in my district.  We
got rid of all the Synoptic LattisHubs and other substandard gear like
that and put in some decent gear.  We have emphatically not regretted it.

Catalyst switches are definitely good stuff, though they are definitely
expensive...if you buy them new.  If I were you, I'd get on eBay and
have a look for the Catalyst 3550, the 48-port version, and make sure
you get the two GBICs along with it.  They very often can be had for
less than US $1,000.

I used to recommend Amer.com's SR48G2i (US $995).  Unfortunately, not
only do they not make that model anymore, but I've also found that the
company is somewhat less than trustworthy.  They made promises to us
that, once they thought they had our money, they demonstrated no
willingness to keep.

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Microsoft Free since 2003 <http://www.cmosnetworks.com>--the ultimate
antivirus protection!

Tom Wolfe wrote:
> Terrell & Julius, thanks for the ideas. The switches I have right now
> are Netgear GS748T (v1). They supposedly support STP but the firmware
> is also a little flakey. I'll give it a shot.
> I'm also definitely into the idea of springing more $$ for better
> switches and moving the Netgears to other less critical locations. The
> Cisco Catalyst switches are expensive -- are they worth the money? Any
> specific recommendations? Any lower cost solutions that are "just as
> good"?
> Regards,
> Tom Wolfe
> ---
>     Tom Wolfe, IT Specialist     twolfe at sawback.com
>     Stoney Education Authority    tel: (403) 881-2650
>     Box 238, Morley AB, T0L 1N0    fax: (403) 881-2793
> Morley Community School | Chief Jacob Bearspaw School | Ta-otha School
> On Thu, 1 May 2008, "Terrell Prud� Jr." wrote:
>> Julius Szelagiewicz wrote:
>>> On Wed, 30 Apr 2008, Tom Wolfe wrote:
>>>> Our Linux labs continue to work great. However, our network has now
>>>> become
>>>> much bigger, with lots of little satellite switches in our mini labs
>>>> (classes that are serviced by a single Cat 5e wire now have 5-15
>>>> K12LTSP
>>>> clients via simple switches).
>>>> 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 like nuts and slows or cripples our network. I then have to
>>>> figure out where the problem is and fix it.
>>>> Are there any recommendations out there on how to prevent these
>>>> problems
>>>> from affecting my whole network, e.g. is there a switch that will shut
>>>> down a port if it's generating too many collisions or problems? And
>>>> maybe
>>>> even email me to alert me of the problem??
>>>> Suggestions would be appreciated!
>>> Many optipons, all good options cost money. The simplest if not least
>>> expensive is to use switches that support spanning tree protocol, end
>>> enable it. There is a small packet delay penalty, but it is usually
>>> negligible. All my HP Procurve switches have STP set and have no
>>> problem
>>> with looped cabling. since this happens in rooms served by a single
>>> cat5
>>> wire, local STP is really crucial. If only a big upstream switch has
>>> this
>>> capability, it will stop all traffic from the affected room.
>>> julius
>> Julius is right.  What you're seeing probably isn't massive collisions,
>> but rather a broadcast storm, which can happen even when everything's
>> Full Duplex (i. e., no collisions).  Several models of switch support
>> STP:
>> HP ProCurve
>> Cisco Catalyst (all models)
>> Amer.com (all managed-switch models)
>> Raptor
>> Avaya
>> I've also used some Nortel BayStack 350-24T and 450-24T switches, which
>> do this job very nicely as well.  They make great LTSP switches, and
>> they're relatively inexpensive on eBay.  Look for one with a Gig-E
>> uplink in it.
>> To get rid of the delay that Julius mentioned, which can take 30
>> seconds, you should configure all your access ports for "Rapid Spanning
>> Tree", or in Cisco parlance, "port-fast" spanning tree.  That'll reduce
>> that delay from 30 seconds down to 2 seconds.  Leave your uplinks on
>> "normal" spanning tree.
>> Additionally, you might want to configure your access ports to do
>> rate-limiting for broadcast frames.  This is relatively easy on any
>> managed switch of decent quality.  We do this at my place of work with
>> rather good results.
>> --TP
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