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Humphrey Legend

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Preventative Measure: MAC Address duplication on VMware

Hi all,

We've had a couple of our VMs with a duplicate MAC-Addresses.
Work Around: Shutdown the VM, set MAC-Address to manual and back to auto, this generates a new MAC time to time.
Is the issue caused by deploying from a Template that has NIC or is there a bug?
>Would removing the NIC from the template be a permanent solution?
>Well, I am trying to figure out how would OTV-EDGE not prevent this from happening across site or even locally for that matter, is the problem on VMware, 'am a bit puzzled.

Avatar of Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)
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Humphrey Legend


we have compared our DEV & Prod VC, the OUI 4th octet is not identical. Also, one of the KBs, reads, probable cause is when moving a VM to a different VC using shared storage, a "COPY-IT" could be the potential probelm, it's either wise to choose to "MOVE-IT" instead.
Copy-it keeps the native MAC and MOVE-IT may not move it.
We picked up a few VMs with a dup-mac, just that we do not want the LAN-Campus Team to point the problem to us whenever there MAC_FLAPs.

Your input is highly appreciated.
It is very rare have you had an issue.

Yes if you are manually copying VMs around and not paying attention to the questions asked if you not read my EE article I'll post it later
We have had a couple of MAC-Flapping, but I doubt this is due to VMware, on the other hand we can't be complacent, hence we started investigating duplicates on our side. In some instances, we would have one business-unit reporting packet-losses....
Cool. please do post it (EE ARTICLE), my interest is dealing with complex problem and the challenge is to put preventative measures to avoid outages.

FYI - We're running UCS.
MAC flapping as Network Admins call it is not caused by duplicate Mac addresses.

Also you WILL be alerted in Vicente to duplicate MAC conflicts in VMs which is rare unless you rapidly deploy 1000s of VMs for VDI

HOW TO: Select the right answer to "I Moved It" or "I Copied It" in VMware vSphere (ESXi)
is MAC-Flapping not a clash of two device interfaces with the same mac-address registered on the switches-A or B and would cause a drop in packets?
MAC Flapping is usually caused by network teaming, the same mac address seen on two different physical port groups at the same time!

this is different to duplicate VM MAC Addresses.
Yep, it makes sense, though I would have thought a mechanism exists to block either or one of the port-groups or disable the port as a form of running a healthier environment.
##The MAC-Address issue on our side is a preventative measure/clean up as we do not want business to complain about intermittent dnetwork connectivity within VMware or be a scapegoat for the network team, we have a become a middleware entity...any issues be it the O/S, Linux, Windows or DBs and Networks dips, we get contacted....but thanks for the clarity
do you require any additional help, to close this question and select a solution.