[124833] in North American Network Operators' Group
Re: what about 48 bits?
daemon@ATHENA.MIT.EDU (Patrick W. Gilmore)
Mon Apr 5 17:27:34 2010
From: "Patrick W. Gilmore" <patrick@ianai.net>
In-Reply-To: <22710.1270501703@localhost>
Date: Mon, 5 Apr 2010 17:26:53 -0400
To: NANOG list <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Apr 5, 2010, at 5:08 PM, Valdis.Kletnieks@vt.edu wrote:
> On Mon, 05 Apr 2010 16:36:26 EDT, Jon Lewis said:
>=20
>> Since they only really need to be unique per broadcast domain, it =
doesn't=20
>> really matter. You can I could use the same MAC addresses on all our =
home=20
>> gear, and never know it. For manufacturers, it's probably reasonably =
safe=20
>> to reuse MAC addresses they put on 10mbit ISA ethernet cards...if =
they=20
>> were a manufacturer back then.
>=20
> Until you buy 25 cards with the same MAC address and deploy them all =
across
> your enterprise
I don't think that's possible given that Jon was suggesting.
I'm 3COM, I made ISA 10Base2 / 10Base5 cards in the 90s. I run out of =
MAC addresses. Instead of going to get more - if I even can! - I =
recycle those MAC addresses, figuring the 10GE PCI-X cards I'm making =
now have 0.000% chance of being on the same b-cast domain as one of =
those old ISA cards.
Even if I am wrong, the max collision possibility is 2, not 25.
Seems reasonable. If I am wrong, I'll apologize profusely, refund the =
price of the 10G card I gave the customer, ship him a new one free, so =
he gets two he can use (assuming he has more than one b-cast domain), =
which would probably make the customer happy. Wanna bet how many times =
3COM would have to ship free 10GE cards?
--=20
TTFN,
patrick
> - 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..)
>=20
> (*) 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. ;)
>=20
> (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'.
>=20
