6.0.1

Table Of Contents
Troubleshooting MAC Address Allocation
In vSphere, certain restrictions on the range of MAC addresses that can be assigned to virtual machines
might cause lost of connectivity or inability to power on workloads.
Duplicate MAC Addresses of Virtual Machines on the Same Network
You encounter loss of packets and connectivity because virtual machines have duplicate MAC addresses
generated by vCenter Server.
Problem
The MAC addresses of virtual machines on the same broadcast domain or IP subnet are in conflict, or
vCenter Server generates a duplicate MAC address for a newly created virtual machine.
A virtual machine powers on and functions properly, but shares a MAC address with another virtual
machine. This situation might cause packet loss and other problems.
Cause
Virtual machines might have duplicate MAC addresses due to several reasons.
n
Two vCenter Server instances with identical IDs generate overlapping MAC addresses for virtual
machine network adapters.
Each vCenter Server instance has an ID between 0 and 63 that is randomly generated at installation
time, but can be reconfigured after installation. vCenter Server uses the instance ID to generate MAC
addresses for the network adapters of the machine.
n
A virtual machine has been transferred in power-off state from one vCenter Server instance to another
in the same network, for example, by using shared storage, and a new virtual machine network adapter
on the first vCenter Server receives the freed MAC address.
Solution
n
Change the MAC address of a virtual machine network adapter manually.
If you have an existing virtual machine with a conflicting MAC address, you must provide a unique
MAC address in the Virtual Hardware settings.
n
Power off the virtual machine, configure the adapter to use a manual MAC address, and type the
new address.
n
If you cannot power the virtual machine off for configuration, re-create the network adapter that is
in conflict with enabled manual MAC address assignment and type the new address. In the guest
operating system, set the same static IP address to the re-added adapter as before.
For information about configuring the network adapters of virtual machines, see the vSphere Networking
and vSphere Virtual Machine Administration documentation.
vSphere Troubleshooting
78 VMware, Inc.