SUMMARY: Two ethernet interfaces in one subnet ?

From: Thomas Leitner (
Date: Mon May 25 1998 - 13:34:31 CDT


The answer this: This is not a good idea because the hostid of the
machine is used for the interface MAC address and both interfaces
thus would have the same MAC address leading to much confusion
with ARP. Since I really want to have all IP addresses in the same
subnet, all I can do is to retire the le0 interface and stick with

Thanks to Marco Greene <> for his answers which
I'm attaching below, followed by my original posting.

Bye now // Tom

From: Marco Greene <>
Subject: Re: Two ethernet interfaces in one subnet ?

    It is not a good idea to put two ethernet interfaces from the same
machine on the same subnet. The reason for this being that the MAC address
is taken care of using the hosts EEPROM. Therefore both interfaces would
have the same MAC address. As you can imagine this was cause some major
problems for ARP.

    However, by the sounds of your question, you are actually putting the
other interface on a different subnet. The machine will automatically
choose the proper route depending on the destination.

Hope this is what your were looking for.

Marco Greene

From: Marco Greene <>
Subject: Re: Two ethernet interfaces in one subnet ?

By the sounds of things, you are phasing out 10 Base-T and going with 100
Base-T. Instead of wasting the existing 10 Base-T switches and cards....I
would use them to make a second LAN. This LAN could simply use a reserved
IP address 10.1.1.x. This is something that I have implemented
for a network backup solution. This way you would not be using your
bandwidth for backups and your general users would be extremely happy as
they don't see the network traffic. The only time they would encounter a
slow down is if they were using a machine that was being backed up at that
point in time.

Just a suggestion to re-use some hardware.

Marco Greene

------------- o r i g i n a l p o s t i n g -----------------------

From: Thomas Leitner <>
Subject: Two ethernet interfaces in one subnet ?


We're in the process of upgrading our workgroup to 100MBit. So I've
ordered a hme interface for our SparcStation 20 running Solaris 2.5.

Now what I have in mind is this:

                        to campus backbone
                      ! Dept. 10Mbit Switch !
                          ! ! le0
      ---------------------------- -----------------
      ! 100Mbit Workgroup Switch ! ! Sparc 20 !
      ---------------------------- -----------------
       ! ! ! ! ! ! ! ! ! ! ! ! ! hme0
       ! ! ! ! ! ! ! ! ! ! ! !--------------
    Machines in our workgroup (new cable)
     (big workgroup server)

The Sparc 20 actually belongs to our workgroup but due to space
constaints it is located in another part of the building and
connected to the 10MBit department switch.

What I have in mind is to run a new cable between our workgroup switch
and the Sparc 20 to the new hme0 interface.

The question is: Does this make sense? Can I configure the Solaris box
in a way that it runs all traffic to the Internet and other campus
machines over the le0 interface but directs all traffic with our
workgroup over the hme0 interface?

All interfaces need to be in the same subnet, though and the main
traffic is NFS where the Sun is an NFS client?

Would a static routing entry like:

   route add host <big-workgroup-server> <name-of-hme-interface>

do this?

Thanks a lot // Tom

T o m L e i t n e r Dept. of Communications
                                            Graz University of Technology,
e-mail : Inffeldgasse 12
Phone : +43-316-873-7455 A-8010 Graz / Austria / Europe
Fax : +43-316-463-697
Home page :
PGP public key on : or send
mail with subject "get Thomas Leitner" to
    Before we have the paperless office, we have the paperless toilet!

This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:12:40 CDT