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Re: IPv6 Link-Local Use Issue for Applications



Bob:

I have encountered duplicate MAC addresses on ethernet NICs on two
occasions: once from Novell and once from IBM.  These incidents were
widely spaced in time: the first approximately ten years ago; the second
about four years ago.  In each case, the problem was uncovered during a
mass deployment of new systems when one machine proved unreliable during
post-install testing. In both cases, the problem was initially
mis-diagnosed as a switch CAM table logic failure.  In one case we found
only one duplicated value (two cards each with the same MAC address); in
the other case we found that the manufacturer had duplicated 16 MAC
addresses, and we got 22 cards with duplicated addresses. (We never did
determine what became of the remaining four cards with duplicate
addresses, although we found 15 of the 16 duplicated addresses in our
stock.)  In each case the manufacturer replaced _all_ the cards out of the
affected production runs with new units, and required that all the cards
we had out of the afficted production run be returned to the maker.

This is not an urban legend, but, based on my experience, manufacturers
tend to react vigorously and very rapidly indeed to remove from public
circulation _all_ cards bearing MAC addresses which may have been
duplicated.

BTW, the networks in which these duplications were discovered are fairly
large; one about 32K hosts, the other about 8K.  I would judge instances
of duplicated MAC addresses to be quite rare.

Hope this helps.

AEB

On Tue, 19 Aug 2003, Bob Hinden wrote:

> Mark,
>
> >b) There have been cases where manufacturers have allocated non-unique MAC
> >addresses. What is worse is that these duplications have apparently tended
> >to happen within the same batch of NICs, and have been encountered when
> >somebody goes to deploy a group of say 20 new NICs they have just bought,
> >and encounter one or more duplications.
>
> Do you have any first hand experience of this happening?  I have heard the
> story several times, but I would like to see something definitive.  I
> wonder if it is an "internet urban myth".
>
> Does anyone have in their possession two or more NICs with the same MAC
> address?
>
> Thanks,
> Bob
>
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-- 
Alan E. Beard <aeb1@aebeard.com>
AEBeard Consulting; 4109 Chelsa Ln; Lakeland FL 33809
863.815.2529


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