Routing Problem (RTFM?) (Summary)

From: Rene Jul-Hansen (
Date: Wed Apr 15 1992 - 14:34:57 CDT

Hello Sun managers,

here is a short summary of the answers I have so far. The general idea
is of course to use a Sun with two Sbus network devices, but it is
actually possible to setup something using Suns with only one network

================================ Original question begin ==================

Is it possible to have two logical internets on one physical net?

  +--------+ +--------+ +--------+
  | host A |-------ethernet-----| Router |------ethernet---| host B |
  +--------+ +--------+ +--------+

Is it possible to connect host A and B if Router is a Sun SLC?

============================= Original question end ======================

summary summary summary summary BEGIN summary summary summary summary

He actually called me on the phone so I'm not quite sure if I got his
e-mail address correct.

He suggested two solutions:

        1) Get a Sun with Sbus and install an extra Sbus device. Use
           this configuration as router.

        2) Take an 'old' PC and use it with two network devices.
           Remove anything that sends out noice (keyboard, disk,
           monitor etc.)
           Use the PC Route public domain package. This should
           be a really good product.


   Sorry, I don't think this will work, but only for subtle reasons.

   I once did this using an old sun3 as the router and all was happy, so
physically it's not a problem putting multiple logical networks on one
physical cable.

   Now here's the GOTCHA 8-):

   Sun has made the assumption that all ethernet boards on a particular
machine are on DIFFERENT physical nets. This allows them the rather
handy trick of conserving ethernet addresses by having the physical
addresses of all interfaces on one machine be the SAME. If you boot
up your SLC (without connecting both ethernet ports to the cable) and
do an ifconfig -a, you should notice that both le0 and le1 will have
the same address (eg 8:0:20:e:f2:74 not internet address).

   I haven't played around with the NVRAM much, but you might check
for some option to separate the ethernet addresses. If it's there,
it's the only way you could make this configuration work. Sorry!


Mario Nigrovic <> voice: (602) 821-4264
Motorola Western MCU Design Center fax: (602) 821-4058


uunet!etnibsd!vsh: His suggestion looked ideal for my test
~~~~~~~~~~~~~~~~~~ purposes. I tried it, and it works great.

re: two networks on a single ethernet cable.

You should be able to. You CAN do it without the router, so you should
be able to do it with. Without a router:

On host A, do:
        /usr/etc/route add `hostname` 0

On host B, do:
        /usr/etc/route add `hostname` 0

or, more logically:

        in /etc/networks on both machines:

                net-1 192.115.52
                net-2 192.115.252

then, On host A, do:
        /usr/etc/route add net-2 `hostname` 0

On host B, do:
        /usr/etc/route add net-1 `hostname` 0

Actually, you can simply have both machines do:

        /usr/etc/route add net-1 `hostname` 0
        /usr/etc/route add net-2 `hostname` 0

since telling the machine that it is the gateway to its own
net is a no-op (I'm pretty sure of this, not posative).

The idea here is to tell each host that IT is the gateway to the
"remote" net, and the hop count is 0. Once this info is loaded into
each host's route table, it will simply put the packet onto the net as
though it were the host's own net.

You can use the /etc/gateways file to achieve the same results if you
wish. And NIS. (This is left as an exercise for the reader :-).

Use netstat -r to view your route table.

.. Steve Harris - Eaton Corp. - Beverly, MA - uunet!etnibsd!vsh

-------------------------------------------------------------------!katkam: ~~~~~~~~~~~~~~~~~~~~~~~

It is possible. It is done by means of examples in the Manual. This is definitely a RTFM.


fuwutai!katkam (Anil Katakam +1 201 386 2524)

------------------------------------------------------------------- ~~~~~~~~~~~~~~~~~~~~~~

What do you mean exactly? Have a machine basically talk two different IP's out the same interface? The answer to this is yes and no. It is possible, in fact, Cisco routers can do this, and route correctly, but Sun doesn't support it.

No. But it would be possible if the router were an SS-1 or other SBUS based machine, and you added another ethernet interface card.

Russ Poffenberger DOMAIN: Schlumberger Technologies UUCP: {uunet,decwrl,amdahl}!sjsca4!poffen 1601 Technology Drive CIS: 72401,276 San Jose, Ca. 95110 Voice: (408)437-5254 FAX: (408)437-5246

-------------------------------------------------------------------!ctm: ~~~~~~~~~~~~~~~~~~~~~~

Yes, you can have two logical internets on the same physical network. I don't think an slc will work as a router though as I don't think it supports a 2nd ethernet port (The only way I know how to set this up is to use two ethernet ports both connected to the same ethernet but each will have an IP address within one of the two networks).

------------------------------------------------------------------- ~~~~~~~~~~~~~~~~~~~

It is possible to have 2 logical nets on one physical net. You do not need a router (for Suns anyway).

If you have machine one with address and machine two with address just do the following commands:

On machine one: route add 0

On machine two:

route add 0

Ted Nolan

-------------------------------------------------------------------!birger: ~~~~~~~~~~~~~~~~~~~~~

I have never tried doing it on a Sun, but using a Cisco router it's possible. The Cisco accepts 'secondary' IP addresses for its ethernet interfaces. I don't think you can do that with the Sun ethernet. But someone on the net may prove me wrong ;)

I have just set up a net with four logical subnets on the same cable using a Cisco with one ethernet card as a router between them. There is no sense in doing this, except in situations where you know the network will split physically as soon as you get the equipment to do it.

Alternatively, you could use two ethernet cards on the same cable. But then you may just as well split them physically. If you would like to try with two cards on the same physical net, remember to set the ethernet address on one of them, as the ethernet address is taken from the host id, and so you would end up with two cards using the same ethernet address. There is an undocumented option to ifconfig to set it. ifconfig le1 ether 8:0:20:f:62:c8 Will set the ethernet address on your second card. BTW: Some see this as a violation of the specs, but it ain't neccesarily so. In fact, one of the ethernet specs says ethernet address should follow host, not card, and that multiple interfaces on the same host *should* use the same address. They should, after all, be on separate physical nets, so using the same ethernet address should do no harm.


------------------------------------------------------------------- ~~~~~~~~~~~~~~~~~

>Is it possible to have two logical internets on one physical net?

I assume you meant "subnets", not "internets". Yes, it's possible, and some systems support multiple IP addresses for an interface.

>Is it possible to connect host A and B if Router is a Sun SLC?

SunOS is not one of those systems, though. I did it with our cisco router for a while. -- Barry Margolin System Manager, Thinking Machines Corp. {uunet,harvard}!think!barmar

------------------------------------------------------------------- ------------------------

You should be able to get A to talk directly to B by putting:

"route add 0" in A's /etc/rc.local, and "route add 0" in B's /etc/rc.local.

See route(8) - the zero hop count for the route indicates that the target logical net is directly connected, so no router is needed. You will need the same sort of thing on each machine on the same cable which needs to communicate with the other logical net.

______________________________________________________________________________ Colin Macleod, Technical Officer, Phone: 0382-23181 x4839 Dundee University Maths & Computer Science Dept. 23 Perth Road, Dundee DD1 4HN, Scotland. EMail:


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Thanks to all who responded. I'll be away on holiday for some time now so if anything interesting should arrive I'll post a post-summary when I get back.

========================================================================== || Rene' Jul-Hansen || Phone : +45-42-800-500 ext 2595 || || Bruel & Kjaer || FAX : +45-42-801-405 || || Test Systems A/S || || || DK-2850 Naerum || E-mail : or || || DENMARK || || |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~| | Fear not tomorrow - tomorrow will look after itself | ==========================================================================

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