(Summary) Two interfaces to the same net on a Sun 4/490 OS 4.1

From: fwp1@cc.msstate.edu
Date: Tue Jul 31 1990 - 22:59:18 CDT

Monday I sent a query about the fact that multi-homed Suns configure the
same ethernet (not internet) address for all interfaces. This was causing
some comfusion on our network.

It seems that Suns have been doing this for quite some time (at least since
3.x). For some reason we weren't bothered by this in earlier releases of
the OS. It may be that an upgrade in some other component on our network
somehow exacerbated the problem (our network is large enough that I cannot
confirm or deny that).

Several people pointed out that you can use the ifconfig statement to set
the ether address thus:

   ifconfig interface ether ##:##:##:##:##:##

Since our primary ethernet number is 8:0:20:8:c2:a5 I set the second interface
to 8:0:20:8:c2:a6 and the problem indeed went away. I don't know how I missed
that option in the manual pages. It isn't very clear but its definitely there.

The only risk I can see to this is the fact that I have no way of guaranteeing
that the next Sun we roll in the door won't want to use 8:0:20:8:c2:a6 as it's
ethernet address. But the chances of this are minute and I don't have any
way to establish a guaranteed unique number (it was suggested that such a
number might have been assigned to the board but I've looked at the board
and the packing information that came with it and can't find one).

Opinions seem to differ upon whether this sharing of numbers is a reasonable
thing to do or not. I have it on good authority (though I don't have the
specs myself) that it is definitely legal (some suggested it wasn't) if you
assume that each interface will be on a separate wire. On the one hand it
can cause this problem if that isn't the case. On the other hand it is
indeed the case almost all the time and definitely saves on ethernet numbers.

My personal opinion is that it is fine as long as there is a sensible way
around it.

Thanks particularly to the following people who answerd my query:

timsmith@Sun.COM (Timothy G. Smith - Technical Consultant)
dupuy@hudson.cs.columbia.edu (Alexander Dupuy)
Kevin N. Carpenter <kncarp@nicsn1.monsanto.com>
blc@sol.med.ge.com (Brett Chapman x5156 )
fabrice@yosemite.atmos.ucla.edu (fabrice cuq)
eplrx7!mcneill@uunet.UU.NET (Keith McNeill)

Frank W. Peters Systems Programmer Computing Center & Services
fwp1@CC.MsState.Edu Peters@MsState.Bitnet (601)325-2942
"I can't give you brains, but I can give you a diploma." -- The Wizard of OZ

This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:05:58 CDT