Path: utzoo!attcan!uunet!cs.utexas.edu!ut-emx!sirius.cc.utexas.edu!spurgeon
From: spurgeon@sirius.cc.utexas.edu (Charles Spurgeon)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet/Optical link locking up
Message-ID: <33599@ut-emx.UUCP>
Date: 9 Jul 90 14:03:22 GMT
References: <1864@trlluna.trl.oz>
Sender: news@ut-emx.UUCP
Reply-To: spurgeon@sirius.cc.utexas.edu.UUCP (Charles Spurgeon)
Organization: Dept. Of Fiddly Bits
Lines: 41

In article <1864@trlluna.trl.oz> gotch@rhea.trl.oz.au (Rob Gotch) writes:
>
>Our problem is that, occasionally, one of the repeated links locks up.
>Close inspection of the equipment reveals that the ANC-20 MAU's at each
>end of the affected link show the collision detect light hard on.
>
>As soon as we disconnect one of the AUI cables to either MAU the
>collision lights go out and upon reconnecting the AUI cable the link
>begins to operate normally - until next time :-(.
>

I recall this happening at Stanford a couple of years ago.  The
symptom was that a DEREP used inside a building between two coax
segments would lock up.  It's been a while, but I seem to recall the
collision light on the MAUs being on solid, as you report.  The MAUs
in the configuration were Cabletron ST-500s.

We tracked the problem down to excessive collisions causing the DEREP
and its attached MAUs to somehow get into what looked like jabber
latch mode.  Power cycling the MAUs (by removing the AUI cables) would
reset things and all was well until the next giant broadcast storm in
the building caused enough collisions to set off the lock up scenario.
Upon investigation, we found several misconfigured hosts with IP
forwarding problems sending huge numbers of broadcasts onto the
building's wire.

One of the folks at Stanford found a way to duplicate the repeater
lock up with a test set up in the lab, using thin wire cable and
several different types of transceivers.  If you injected packets into
the system, and then unterminated the thin net segments in the right
order, you could get the DEREP and its (non-DEC) MAUs to lock up.

The DEREP problem was "solved" by using DEC H4000 transceivers with
the DEREPs.  As I recall, we were told that only the H4000s are
guaranteed to work with DEREPs.


Charles Spurgeon                     |                                  |
UTnet Network Information Center     | spurgeon@emx.utexas.edu          |
University of Texas at Austin        | ...!cs.utexas.edu!ut-emx!spurgeon|
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