two interfaces one subnet

Chris Meidinger cmeidinger at
Mon May 11 15:45:53 CDT 2009

On 11.05.2009, at 22:34, Patrick W. Gilmore wrote:

> On May 11, 2009, at 4:29 PM, Chris Meidinger wrote:
>> I would be grateful for a pointer to such an RFC statement,  
>> assuming it exists.
> Why would an RFC prohibit this?
> Most _implementations_ do, but as far as network "rules" in general  
> it is a valid configuration.

That was essentially my conclusion as well: logically it can't work,  
but I wasn't certain where it might be forbidden.

Thusly did I come to NANOG with the question, thinking smarter people  
than I might know. If it's completely down to implementation, or  
really to the interaction between TCP and underlying IP, then so be  
it. I was hoping that I might just not have thought of the right place  
to look.

On 11.05.2009, at 22:39, Mikael Abrahamsson wrote:

> On Mon, 11 May 2009, Chris Meidinger wrote:
>> 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.
> I don't know if it still works, but it did in Linux little over 10  
> years back. Proxy-arp:ed all the IPs in the /27 in the /24 and  
> everything was fine (legacy reasons plus radiolink which I didn't  
> want to run a lot of broadcasts over). There are "legitimate" cases  
> where you might want to do this.

Yes, I've gotten it to work as well as little as 10 days ago, but it's  
not something that $random_customer should be doing as a matter of  

Thus, again, my hope that I just wasn't thinking of the right place to  
look to find an IETF recommendation against doing so.

Thanks for the input!


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