Router with 2 (or more) interfaces in same network

Leo Bicknell bicknell at ufp.org
Tue Nov 11 14:55:34 UTC 2003


In a message written on Tue, Nov 11, 2003 at 08:35:34AM +0000, Sugar, Sylvia wrote:
> I am curious to know if its possible to have a router with its two interfaces, say configured as, 
> 1.1.1.1/16 and 1.1.1.2/16. Theoretically, i see nothing which can stop a router from doing this.

Cisco's don't let you do this.  I have always considered that broken,
although I'm sure Cisco thinks it's a feature.  Other routers (of
note FreeBSD boxes) do this just fine.  In almost all cases I've
seen it done it was for more bandwidth to the box (typically inbound
only, because there are no good tools on Unix boxes to split the
traffic between the outgoing interfaces).  I've seen it done a lot in
labs where you have something like this:

client 1 |         | client 5
client 2 +----B----+ client 6
client 3 |         | client 7
client 4 |         | client 8
         |         |
    file-server-router-box
              |
          Internet

Where all the clients are in one subnet, there are two interfaces,
and the networks are separated (today the left and right groups on
two different switches, I drew the old school picture of thinwire
with a bridge in the middle.

While this will work (with some boxes, again Cisco's won't let you
configure the same subnet on two interfaces), it is at best a hack
that helps in some specific instances.  It is quite clearly not good
network design.  Maybe they have one of those specific instances
but I'd get a lot more detail and be sure before you offer up this
hack as otherwise you've got a messy config that didn't do what the
customer wanted anyway.

-- 
       Leo Bicknell - bicknell at ufp.org - CCIE 3440
        PGP keys at http://www.ufp.org/~bicknell/
Read TMBG List - tmbg-list-request at tmbg.org, www.tmbg.org
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