two interfaces one subnet

Matlock, Kenneth L MatlockK at exempla.org
Mon May 11 16:50:42 CDT 2009


If it were me and had the requirement of having both NICs in the same L2
segment, but unique IP addresses, I'd assign a secondary IP address to
the Layer3 SVI on the upstream device, and give the 2nd NIC on the
server an IP on that secondary IP block. 

Ken Matlock
Network Analyst
Exempla Healthcare
(303) 467-4671
matlockk at exempla.org
-----Original Message-----
From: Chris Meidinger [mailto:cmeidinger at sendmail.com] 
Sent: Monday, May 11, 2009 3:39 PM
To: Dan White
Cc: nanog at nanog.org
Subject: Re: two interfaces one subnet

On 11.05.2009, at 23:31, Dan White wrote:

> Chris Meidinger wrote:
>> 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.
>
> If your goal is to achieve redundancy or to increase bandwidth, you  
> can bond the interfaces together - assuming that you have a switch /  
> switch stack that supports 802.3ad.
>
> Then you could assign multiple IPs to the bonded interface without  
> any layer 3 messyness.

I should have been clearer. The case in point is having two physical  
interfaces, each with a unique IP, in the same subnet.

For example, eth0 is 10.0.0.1/24 and eth1 is 10.0.0.2/24, nothing like  
bonding going on. The customers usually have the idea of running one  
interface for administration and another for production (which is a  
_good_ idea) but they want to do it in the same subnet (not such a  
good idea...)

Chris





More information about the NANOG mailing list