Re: [Hampshire] One box, 2 NICs - but with the same MAC addr…

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Author: David Ramsden
Date:  
To: Hampshire LUG Discussion List
Subject: Re: [Hampshire] One box, 2 NICs - but with the same MAC address

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On Wed, Jan 31, 2007 at 09:49:42AM -0000, Bond, Peter wrote:
> The idea makes me cringe, but someone has decided that it would be a "good" idea (for certain values of good) to have 2 network ports on the same system with the same MAC & IP addresses (well, they'll never be on the same physical network)... I don't think this is sane, sensible or conforming to general networking principles; am I being fair to refuse it? I can see all sorts of problems, and I maintain that it is the responsibility of an upstream router to perform any NAT that is required (along with the redundancy switching).
>


Hi Peter,

I don't quite understand what is trying to be acheived. Is this to
implement a failover solution? If one network card fails, you can go and
physically unplug the cable from one NIC and place it in the other and
then ifup the interface?

What was the reasoning behind giving both NICs the same MAC address? The
only reason I can think of is either your switches and/or routing
equipment have been implemented to only allow access from known MAC
addresses or to try to reduce the downtime if a NIC fails because if you
unplug one NIC and plug in another with the same IP address, ARP tables
won't have timed out and the box won't be seen on the network.

If I've not understood the requirement then the rest of this email can
probably be ignored. :-)

> And if I'm being more-than-usually unreasonable, is there a way of achieving it without rewriting chunks of the stack?
>


If my understanding is correct then this sounds like a horrible solution
and will introduce more administrative overhead.

You want to look in to bonding, where you can have two NICs for example,
both plugged in to the same physical network. When bonding has been
implemented you will see one logical interface. However, if one NIC
fails everything should continue to work as normal. Note the *should*
:-) You may find you need switches that support link aggregation.

There is plenty of documentation available for this on Google and also
by reading Documentation/networking/bonding.txt found in the kernel
source.

Regards,
David.
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