fatbobsufc
Contributor
Contributor

Should I be trying the lotto?

What's the chances of this? we had a Mac address clash on our network. on investigation it turned out to be a Windows 2003 VM which clashed with the MAC from a physical SUN server.

Would have thought

A. VMware reserved a block of MAC addresses which should differ to those reserved by SUN and

B. the chances of this kind of clash on two unrelated machines would just be too remote!

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5 Replies
jb12345
Enthusiast
Enthusiast

Years ago we had a MAC conflict with 2 physical machines. It took weeks to find the machines. The problem was caused because we had a NIC in one machine, I won't mention the brand but it was at that time a first tier brand, that was actually the same as another of the same brand. What had happened was that the manufacturer had purchased another manufacturer and their inventory and just changed the manufacturer part of the MAC address so they ended up with several hundred duplicate address cards.

Moral - if it can happen, it will eventually.

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dominic7
Virtuoso
Virtuoso

So what was the MAC address? VMs don't generally have static MAC addresses so this would be extra tough to track down. Was there a hard coded MAC on the VM, or was it in the VMware range?

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fatbobsufc
Contributor
Contributor

This was reported to me by someone in my team. The mac addresses are now dissimilar so I suspect one of two things:

  1. the miraculous clash occurred by chance and the mac address has now dynamically changed to another after a reboot

  2. There was an error in the routing tables on our passport switch (which did show similar mac addresses for the two hosts but now does not - after routing tables flush?)

Either way - problem seems to have gone away. I will keep my eye on the situation Smiley Happy

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dominic7
Virtuoso
Virtuoso

You can mitigate this problem by changing the security settings on your port group to not allow mac changes which will restrict your VMs from having their MAC changed by the guest opertating system. Then if you find an offending system, they're stomping on VMware territory which would be a problem with the other system and not your VMware environment. I wrote up a blog about MAC changes and your options in cloning MAC addresses a while back:

http://vmprofessional.com/2009/06/mac-address-cloning.html

-Dominic

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Rumple
Virtuoso
Virtuoso

Not related to vmware, but I was at a site that bought over 500 palm handhelds. We used MAC to allow authentication to the network...imagine our suprise during rollout when 20% of the Palm's had duplicate MAC's :_|

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