two interfaces one subnet

Patrick W. Gilmore patrick at
Mon May 11 22:01:01 CDT 2009

On May 11, 2009, at 8:04 PM, Ben Scott wrote:
> On Mon, May 11, 2009 at 6:01 PM, Patrick W. Gilmore  
> <patrick at> wrote:

>> It doesn't matter which physical interface transmits the packet.
>  Well, in the general sense, I suppose not.  The computer can put
> whatever it wants in an Ethernet frame, and as long as it's valid for
> the receiving system, it will work.

The OP asked for an RFC showing why this is forbidden.  It is not.  It  
works fine.  I stated many times that several implementations deny  
your ability to do this, but the "rules" permit it just fine.

>> Another example: Imagine a web server with two uplinks in _different_
>> subnets running Quagga.
>  That's a different scenario entirely.  Diverse routes work fine
> because all the intermediate routers work the same way I describe
> above: They don't care where the packet came from, they don't know
> about "connections", they just forward packets to the destination.

Do you even read your own posts?  Specifically:

On May 11, 2009, at 5:40 PM, Ben Scott wrote:

> Either way, if
> the packet *from* X was addressed *to* B but the response comes back
> from *A*, then host X is going to drop the packet as
> invalid/irrelevant/etc.

The receiving host X does not care (or even know) if A and B are in  
the same prefix.  Intermediate systems have nothing to do with it as  
they do not touch the source IP address in the packet.  (We are  
obviously ignoring NAT/PAT, etc.)  So if it works with "diverse  
routes", it works without diverse routes.

In other words, you contradicted yourself.  Don't worry, you are in  
good company in this thread.  The OP alone did that 4 times by my  
count, and I stopped reading his posts because he did it so often.

To summarize: Two physical interfaces on one machine in the same  
prefix is allowed.  There is no RFC against it - just the opposite.   
So quit arguing over "but my $THING doesn't support it properly" or  
"but it will break $SOMETHING" or whatever your favorite hang-up is.


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