Sort of SUMMARY: Multiple IP-Interfaces in one IP-Subnet on one host

From: Wolfgang Kuehnel (
Date: Mon Mar 08 1993 - 08:43:38 CST

This is what I recieved up to Monday, March 8, 1993. I'll will give
more details later (when I return from skiing. :-)
I was told that there already was discussion about this topic, including
a summary.
Christian Lawrence <> would like to see some
final clues. So I keep this relatively short and give some longer
report later when I found time to work on that.

My query was:
$ What I want to do is to add these additional interfaces ON THE SAME
$ IP SUBNET. I dont want to split the workgroup into multiple subnets,
$ for cabling reasons.
$ I just want to split the clients in the netgroup in one half, which
$ mounts its filesystems from, and the other ones from
$ Two different IP-numbers, but same host. At first
$ glance, each interface has to handle half of the traffic.
$ My questions are:
$ Who has tried this before (and failed :-) ?
$ Will this give the desired performance improvement at all ?
$ Will the kernel *receive* every broadcast message (ICMP) twice ?
        Will the machine *send* every broadcast message twice ?
$ Which interface will be used when the server sends IP packets ?

The answers and opinions fell into three categories:
- Can not be done
- Can be done, but it is tricky and a lot of work
- Can be done, but wont improve anything

At first, Sun re-configures the MAC-address of second, third.... interface
to be the same as the first one. This is done for conserving address space
or to be compliant with their routing code, there weretwo different opinions.
To connect two ethernet interfaces with the same address to a single segment
is not allowed. This problem can be solved by re-programming the MAC-address
of the additional interfaces by hand (ifconfig) at boot-time in rc.* or,
as I did, using one interface for DECnet, what also re-configures the
MAC-address. The second solution has the advantage that SUN will support it,
with the first they will leave you alone with your problems.
This is independent from the SunOS version.

Second, you have to consider routing. Ed Strong <> said
it may be possibple and supported by SUN in SunOS 4.1.3. Dont know about
SysVR4 Solaris. Others said that its impossible to do with Suns routing
algorithm, its better to get gated (if you prefer dynamic routes and have
time to experiment) or add static routes, to tell the machine which
interface to use for out-bound traffic. RIP does not support such a
configuration, will have to use IGRP or OSPF for support of multipath routing.

I tried it out (on a 4.1.3 sun4 with an in.routed) in the meantime,
this is what happend:
Immediately after assinging an IP-address to the second interface, the
route via the first one to the subnet the machine was on was deleted
from the kernels tables. The routes to the other subnets were not affected.
No matter, we had a new route, so that was OK.
I added a static route to the attached subnet via the first interface.

I reconfigured some clients of the machine to NFS via the new interface,
that worked OK. I added static routes to these hosts via that new
interface. The server will run in this configuration for serveral days,
I'll do some statistics about network performance and post to the list.

Thanks again for your time and help.


As I saw in recent summaries it seems to be etiqeutte to explicitly
thank those who answered. So I will do, not only it's etiquette but
because I really appreciate your help. Thanks.

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