two interfaces one subnet

Chris Meidinger cmeidinger at
Mon May 11 16:49:22 CDT 2009

On 11.05.2009, at 23:42, Kevin Oberman wrote:

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

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.


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