what about 48 bits?

Valdis.Kletnieks at vt.edu Valdis.Kletnieks at vt.edu
Mon Apr 5 16:08:23 CDT 2010


On Mon, 05 Apr 2010 16:36:26 EDT, Jon Lewis said:

> Since they only really need to be unique per broadcast domain, it doesn't 
> really matter.  You can I could use the same MAC addresses on all our home 
> gear, and never know it.  For manufacturers, it's probably reasonably safe 
> to reuse MAC addresses they put on 10mbit ISA ethernet cards...if they 
> were a manufacturer back then.

Until you buy 25 cards with the same MAC address and deploy them all across
your enterprise - the problem can go un-noticed for *weeks* as long as two
boxes aren't squawking on the same subnet at the same time(*).  Of course, you
never stop to actually *check* that two cards in different machines have the
same address, because That Never Happens, and you spin your wheels trying to
figure out why your switching gear is confused about the MAC addresses it's
seeing (and it always takes 3 or 4 tickets before one actually includes the
message "Duplicate MAC address detected" in the problem report..)

(*) And as Murphy predicts, whenever it happens, one of the two offenders will
give up in disgust, power off the machine, and go on coffee break so the arp
cache has timed out by the time you start trying to work the trouble ticket. ;)

(Yes, we're mostly older and wiser now, and more willing to include "the damned
hardware is posessed by an Imp of Perversity" in our troubleshooting analysis.
Had an SL8500 tape library last week that reported 'Drive State: Unpowered' and
'Drive Status: Not Communicating' and still reported 'Drive Health: Good'.

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