[114457] in North American Network Operators' Group
RE: two interfaces one subnet
daemon@ATHENA.MIT.EDU (Holmes,David A)
Mon May 11 18:32:14 2009
Date: Mon, 11 May 2009 15:27:13 -0700
In-Reply-To: <4B3726C0-20FE-4598-B4DA-44FA74FDFC71@sendmail.com>
From: "Holmes,David A" <dholmes@mwdh2o.com>
To: "Chris Meidinger" <cmeidinger@sendmail.com>
Cc: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
I think the idea of one interface per subnet originates in the early
RFCs, such as RFC 1009 "Requirements for Internet Gateways":
"Section 1.1.2 Networks and Gateways
... A gateway is connected to two or more networks, appearing to
each of these networks as a connected host. Thus, it has a
physical interface and an IP address on each of the connected
networks ... "
So by using singular terminology ( "a connected host", "a physical
interface", "an IP address") instead of plural, a single interface per
subnet for gateways (read routers) is implied.
This is not to say that it will not work, at least on servers. Standards
aside, a good reason why this is not a best practice is the concept of
asynchronous routing where a packet arrives on one interface, and the
reply leaves on the other interface with a different source IP on the
reply. Most firewalls will reject packets such as this. =20
-----Original Message-----
From: Chris Meidinger [mailto:cmeidinger@sendmail.com]=20
Sent: Monday, May 11, 2009 1:29 PM
To: nanog@nanog.org
Subject: two interfaces one subnet
Hi,
This is a pretty moronic question, but I've been searching RFC's on-=20
and-off for a couple of weeks and can't find an answer. So I'm hoping =20
someone here will know it offhand.
I've been looking through RFC's trying to find a clear statement that =20
having two interfaces in the same subnet does not work, but can't find =20
it that statement anywhere.
The OS in this case is Linux. I know it can be done with clever =20
routing and prioritization and such, but this has to do with vanilla =20
config, just setting up two interfaces in one network.
I would be grateful for a pointer to such an RFC statement, assuming =20
it exists.
Thanks!
Chris
