two interfaces one subnet

Mike O'Connor mjo at dojo.mi.org
Mon May 11 16:39:51 CDT 2009


: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.

RFC1122, Section 3.3.4.1 explicitly says this IS a legal config
from an IP perspective:

      3.3.4  Local Multihoming

         3.3.4.1  Introduction

            A multihomed host has multiple IP addresses, which we may
            think of as "logical interfaces".  These logical interfaces
            may be associated with one or more physical interfaces, and
            these physical interfaces may be connected to the same or
            different networks.

There are other considerations here -- OS, link-layer, etc.
Obviously, you want to do such things with care.  But simply
from a "standards" perspective, it's ok.  There are a lot of
hosts that historically didn't have enough RFC1122 compliance
to make such configurations problematic (e.g. section 3.3.1.2
and multiple default route support vs. old BSD IP stacks) but
that doesn't invalidate the standards.

-- 
 Michael J. O'Connor                                          mjo at dojo.mi.org
 =--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--==--=
"Pain has an element of blank."                              -Emily Dickinson




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