vSphere 5.1 and later provides several schemes for automatic allocation of MAC addresses in vCenter Server. You can select the scheme that best suits your requirements for MAC address duplication, OUI requirements for locally administered or universally administered addresses, and so on.
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After the MAC address is generated, it does not change unless the virtual machine's MAC address conflicts with that of another registered virtual machine. The MAC address is saved in the configuration file of the virtual machine.
Preventing MAC Address Conflicts
The MAC address of a powered off virtual machine is not checked against the addresses of running or suspended virtual machines.
When a virtual machine is powered on again, it might acquire a different MAC address. The change might be caused by an address conflict with another virtual machine. While this virtual machine has been powered off, its MAC address has been assigned to another virtual machine that has been powered on.
If you reconfigure the network adapter of a powered off virtual machine, for example, by changing the automatic MAC address allocation type or setting a static MAC address, vCenter Server resolves MAC address conflicts before the adapter reconfiguration takes effect.
For information about resolving MAC address conflicts, see the vSphere Troubleshooting documentation.
VMware OUI Allocation
VMware Organizationally Unique Identifier (OUI) allocation assigns MAC addresses based on the default VMware OUI 00:50:56 and the vCenter Server ID.
VMware OUI allocation is the default MAC address assignment model for virtual machines. The allocation works with up to 64 vCenter Server instances, and eachvCenter Server can assign up to 64000 unique MAC addresses. The VMware OUI allocation scheme is suitable for small scale deployments.
MAC Address Format
According to the VMware OUI allocation scheme, a MAC address has the format 00:50:56:XX:YY:ZZ where 00:50:56 represents the VMware OUI, XX is calculated as (80 + vCenter Server ID), and YY and ZZ are random two-digit hexadecimal numbers.
The addresses created through the VMware OUI allocation are in the range 00:50:56:80:YY:ZZ - 00:50:56:BF:YY:ZZ.
Prefix-Based MAC Address Allocation
On ESXi hosts 5.1 and later, you can use prefix-based allocation to specify an OUI other than the default one 00:50:56 by VMware, or to introduce Locally Administered MAC Addresses (LAA) for a larger address space.
Prefix-based MAC address allocation overcomes the limits of the default VMware allocation to provide unique addresses in larger scale deployments. Introducing an LAA prefix leads to a very large MAC address space (2 to the power of 46) instead of an universally unique address OUI which can give only 16 million MAC addresses.
Verify that the prefixes that you provide for different vCenter Server instances in the same network are unique. vCenter Server relies on the prefixes to avoid MAC address duplication issues. See the vSphere Troubleshooting documentation.
Range-Based MAC Address Allocation
On ESXi hosts 5.1 and later you can use range-based allocation to include or exclude ranges of Locally Administered Addresses (LAA).
You specify one or more ranges using a starting and ending MAC addresses, for example,(02:50:68:00:00:02, 02:50:68:00:00:FF). MAC addresses are generated only from within the specified range.
You can specify multiple ranges of LAA, and vCenter Server tracks the number of used addresses for each range. vCenter Server allocates MAC addresses from the first range that still has addresses available. vCenter Server checks for MAC address conflicts within its ranges.
When using range-based allocation, you must provide different instances of vCenter Server with ranges that do not overlap. vCenter Server does not detect ranges that might be in conflict with other vCenter Server instances. See the vSphere Troubleshooting documentation for more information about resolving issues with duplicate MAC addresses.
Info taken from VMware Documentation
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