two interfaces one subnet

Kevin Oberman oberman at
Mon May 11 16:42:40 CDT 2009

> Date: Mon, 11 May 2009 16:19:56 -0500
> From: "Alex H. Ryu" <r.hyunseog at>
> Unless you configure Layer 2 for two interfaces, it's not going to work.
> It is invalid from networking principle.
> If you have to send the traffic for host in same subnet you configured,
> which interface it should send out ?
> Basically it may create broadcast storm loop by putting two ip addresses
> in same subnet in different interface.
> It may be allowed from host-level, but from router equipment, I don't
> think it was allowed at all.
> Alex

I am a bit baffled as to why people think:
1. It won't work
2. It is a bad thing to do if it would work

Neither is true. If it is two separate interfaces with two MAC
addresses, it will work fine IF one of the interfaces is configured with
a netmask of (/32). Of course, you will have to add
routes for the second interface if you expect to source traffic from it,
but it really in not rare.

Many network devices are intended to be configured this way. NetApp was
mentioned, but it is not unique. Doing this is expressly covered in the
documentation for FreeBSD.
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751

> Chris Meidinger wrote:
> > Hi,
> >
> > This is a pretty moronic question, but I've been searching RFC's
> > on-and-off for a couple of weeks and can't find an answer. So I'm
> > hoping someone here will know it offhand.
> >
> > I've been looking through RFC's trying to find a clear statement that
> > having two interfaces in the same subnet does not work, but can't find
> > it that statement anywhere.
> >
> > The OS in this case is Linux. I know it can be done with clever
> > routing and prioritization and such, but this has to do with vanilla
> > config, just setting up two interfaces in one network.
> >
> > I would be grateful for a pointer to such an RFC statement, assuming
> > it exists.
> >
> > Thanks!
> >
> > Chris
> >
> >
> >

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