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From: ercm20@castle.ed.ac.uk (Sam Wilson)
Newsgroups: comp.dcom.sys.cisco
Subject: Re: all of the ethernets interfaces with the same address!?
Message-ID: <9770@castle.ed.ac.uk>
Date: 22 Apr 91 20:52:27 GMT
References: <33874@boulder.Colorado.EDU> <1991Apr3.222527.3422@ultra.com> <1991Apr21.170045.21393@pcserver2.naitc.com>
Organization: Edinburgh University
Lines: 35

In article <1991Apr21.170045.21393@pcserver2.naitc.com> kdenning@pcserver2.naitc.com (Karl Denninger) writes:
>In article <1991Apr3.222527.3422@ultra.com> shj@ultra.com (Steve Jay) writes:
>>In <33874@boulder.Colorado.EDU> BILLW@mathom.cisco.com (WilliamChops Westfield) writes:
>>
>>> [ controversy over whether it is ever done to set the MAC address of
>>>   all ethernet ports on a machine to be the same ]
>
>It is true on SOME machines -- primarially those which get their ethernet
>address from the system PROM rather than from the board's PROM.
>
>>From my neighborhood Sun server:
>> [ example of Sun ifconfig showing the same MAC address for 2 i/fs ]
>
>Note that you can change that with "ifconfig ie1 ether xx:xx:xx:xx:xx:xx"
>
>Not all values are valid for the hardware address, but enough are to make it
>useful. (I usually change the "20" to "21").

The only systems that I'm aware of that regularly do this are DEC
machines running DECnet (which have to because of the way DECnet phase
IV works) and Suns.  It seems that Sun have an ideosyncratic reading of
the IEEE standard which says that each 802.x station must have a unique
MAC address - they take 'station' to mean each computer rather than each
interface and set all MAC addresses the same.  This has interesting
implications for mutihomed stations in bridged networks...  :-). 

If you're planning to change the MAC address I believe you ought to set
the 'locally administered' bit - bit 1 (value 2) in the first octet,
thus a modified Sun address might start 0a-00-20 rather than 08-00-20,
but you can set it to anything you like so long as you don't set the low
order bit of the first octet, the multicast bit.  DEC do it that way:
DECnet MAC addresses start aa-. 

Sam Wilson
Network Services, Edinburgh University Computing Service, Scotland, UK