two interfaces one subnet

Date: Mon, 11 May 2009 16:19:56 -0500
From: "Alex H. Ryu" <>

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.


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.

This is, of course, how I've done it at times in the past. Routing management can, however, become quite a pain over time.

The customer expectation is, naturally, that any traffic related to a connection that comes in to the first interface should go back out that interface, and anything related to a connection that came into the second interface should go back out there. (All this without any specific routing etc.)

I think we both know that that's not going to happen automagically.