vCenter MAC Address allocation and conflicts

Fridays in the Xtravirt office offers a time for consultants to gather round and have a great chinwag about whats been going on for them in the week and, more importantly, ask questions and get answers! Something raised this week was the allocation of MAC addresses to virtual machines. If you have multiple vCenter instances creating virtual machines, how can you guarantee that you wont get conflicts generated? It’s an interesting question and the answer is documented; but you may not already know the answer.

When a vCenter server is installed a unique ID is generated. This ID is randomly generated and is in the range of 0 to 64. The ID  gets used when generating MAC address and the UUIDS, or unique identifiers for virtual machines. You can see that if two vCenter servers had the same unique ID, a possibility exists that duplicate MAC addresses might get generated; cue packet loss, connectivity issues and your desk phone ringing a lot…

So what do we do?

Solution: If you deploy virtual machines from multiple vCenter Server systems to the same network, you must ensure that these vCenter Server systems have unique instance IDs. How can you check the ID of each vCenter server? It’s simple.. login to vCenter, and go to Administration, vCenter Server Settings. Click Runtime settings and you can see the unique ID.

Simply, do this on each vCenter server. If you have a duplicate ID, change it! Just dont forget to restart the vCenter services afterwards!