vFrank

Essense of virtualization

MAC address conflicts

by 6 Comments

During a VMware Install, Configure Manage class I had two students each with seperate vCenters complaining about their virtual machines were receiving the same IP address from the DHCP server. At first I thought there was some weird problem with my DHCP server but that was not the case. We checked the VMware generated MAC addresses for the problem VM’s and guess what… they had the same MAC addresses. That pretty much told us why they were having IP conflicts.

At that time I was not aware how vCenter generates is MAC addresses but after asking google and kb.vmware.com I figured it out.

Every vCenter has a Unique ID. (administration – vCenter server settings – Runtime settings) The value can be between 0 and 63. The value is generated during the installation of vCenter. I checked both vCenters and they had the same unique ID 2.

vCenter then uses the following formula for MAC address generation: 00:50:56: (80+UniqueID) :00:00 This means it is only the last four digits

vCenter changes.

I got more curious about this and tried logging directly in to an ESXi host and creating two virtual machines. They got the following MAC addresses

VM0 00:0C:29:5C:16:BE

VM1 00:0C:29:73:B2:6F

During my research I disovered VMware have three OUI for MAC addresses

00:50:56 (Used for vCenter)

00:0C:29 (Used when creating VM directly on an ESXi host)

00:05:69 (Maybe used on ESX?)

When going through the VMware knowledge base there was a document stating that you should use 00:50:56 for static MAC address. Use the range 00:50:56:(00-3F) for your static mac addresses

  • Hi Frank,

    It is safe to use static MAC addresses through vCenter that start with 00:50:56 – the fourth byte of a static MAC address must be in the 00-3F range.

    Try it for yourself in the Edit Settings dialog box for a VM in the vSphere Client 😉

    Scott.

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  • Thanks Scott for the info! 🙂

  • Hey Frank
    I was a student during your class. Something doesn’t fit:
    I have created a VM in my test lab. It was given the MAC Address: 00:00:56:b7:00:00. vCenter have the Unique ID 55. According to you the fourth byte should be 80+55 =135 (DEC) or 87(HEX) but it is b7 (HEX). If 80+55 are read as HEX it doesn’t help. It gives 213 (DEC) or D5(HEX)

    Congratulations with your exam. In the next week it is my turn with VCP 🙂
    Gert

  • Hey again

    It does fit if I read 80 as HEX and 55 as DEC :-). I didn’t take that possibility in consideration.

    Gert

  • Good luck with the VCP let me know how it goes 🙂