From owner-freebsd-hackers  Tue Apr 28 14:50:41 1998
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Message-ID: <19980427230813.00205@mi.uni-koeln.de>
Date: Mon, 27 Apr 1998 23:08:13 +0200
From: Stefan Esser <se@FreeBSD.ORG>
To: =?iso-8859-1?Q?Dag-Erling_Coidan_Sm=F8rgrav?= <dag-erli@ifi.uio.no>,
        "Matthew N. Dodd" <winter@jurai.net>
Cc: Hans Huebner <hans@artcom.de>, freebsd-hackers@FreeBSD.ORG,
        Stefan Esser <se@FreeBSD.ORG>
Subject: Re: FreeBSD HA configuration / Ethernet address takeover
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	"Matthew N. Dodd" <winter@jurai.net>, Hans Huebner <hans@artcom.de>,
	freebsd-hackers@FreeBSD.ORG, Stefan Esser <se@freebsd.org>
References: <Pine.BSF.3.96.980426233709.21511D-100000@sasami.jurai.net> <xzpra2jfxkl.fsf@hrotti.ifi.uio.no>
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On 1998-04-27 11:00 +0200, Dag-Erling Coidan Smørgrav  <dag-erli@ifi.uio.no> wrote:
> "Matthew N. Dodd" <winter@jurai.net> writes:
> > I'm kind of puzzled at how this would be integrated with the
> > SIOCGHWADDR/SIOCSHWADDR calls as you might also need a way of determining
> > which hadware address to set/get :)
> 
> No. You can set any arbitrary MAC address, but it will make a heavy
> impact on network performance, since much of what is usually done in
> hardware (discarding packets not meant for you) will have to be done
> in software.

No, not true. Ethernet chips usually don't hardwire 
the MAC address, but provide one (or more) register(s)
that typically get initialised from a serial EEPROM.
But that register may be written to, and there are
Ethernet protocols that rely on that feature (DEC LAT, 
DECnet).

Many Ethernet chips accept 16 addresses, including
Multicast addresses. In addition to exact address
matches, there is often a hash (using the Ethernet
CRC algorithm and hardware) used to select N out of 
64 to N out of 512 MAC addresses, leaving the exact 
address match to be done in software.

Regards, STefan

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