[AusNOG] The occurrence of duplicate MAC addresses

[Mark Smith]

>________________________________
> From: Lincoln Dale <ltd at aristanetworks.com>
>To: Gavin Tweedie <gav at narx.net> 
>Cc: ausnog at lists.ausnog.net 
>Sent: Friday, 8 February 2013 7:22 PM
>Subject: Re: [AusNOG] The occurrence of duplicate MAC addresses
> 
>
>On Fri, Feb 8, 2013 at 6:07 PM, Gavin Tweedie <gav at narx.net> wrote:
>
>
>>Bad luck for guy #2 who rocks up to the peering point with the same MAC as an existing member eh? Or potentially bad luck for both if nobody notices before they connect.
>>
>
>best practice in any peering point is to disallow mac-moves like that - so should not cause an issue for 1st person with mac-address.
>
>should not be a real problem for the 2nd person either as any clueful network equipment pretty much allows you to set the mac-address if you so desire anyway.
>
>  switch#conf t
>  switch(config)#int et17
>  switch(config-if-Et17)#mac-address ?
>    H.H.H  MAC address
>
> 
>
>
>>An interesting stat to know would be what sort of vendor equipment this is occurring on, is it just low end home gateway devices or does the vendor have the same issue on higher end gear?
>>
>
>i say its a storm in a teacup.
>even if its a NIC, you can override the MAC address anyway on just about anything.


You might be able to, but could your mother (or any other non-technical relative), even under instruction over the phone? What if the device doesn't save the new MAC address, such that it has to be changed every time the device boots?

Global uniqueness is intended for "plug and play" convenience, rather than a functional requirement. I went looking a number of years ago as to why MAC addresses are so large when they really only needed to be around a maximum of 16 bits. Here's the paper on the topic, which also has good discussion on addressing in general:

"48-bit Absolute Internet and Ethernet Host Numbers"
http://ethernethistory.typepad.com/papers/HostNumbers.pdf





>
>
>cheers,
>
>lincoln.
>
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