several virtual machines all with the same mac address?

[marco201110142]

Hi,

I'd like to know if with VMware Workstation 7.1.3 is possible to create several virtual machines all with the same mac address (but different IP) and with bridged network connection. Virtualbox allows this configuration.

[kjb007]

While a generated mac address will be unique, you can use create custom MAC addresses, and assign them to virtual machines.  I haven't done this myself, so I'm not sure if workstation will nag at you because of it.  The kb article that describes the change process is below.

http://kb.vmware.com/kb/507

[marco201110142]

I have already tried to assign the same mac address to 2 virtual machines. When the second VM is powered on I get an error message (the mac address is already used or reserved...).

I'm not sure it is possible with VMWare. On the other hand, Virtualbox allows to do that but I have performance issues with the latter...

[kjb007]

This entry may be of help.

ethernet0.ignoreMACAddressConflict                = "false"

http://sanbarrow.com/vmx/vmx-network-advanced.html#mac

[bulletprooffool]

you'll need to edit the VMX file to read :

ethernet0.ignoreMACAddressConflict                = "TRUE"

I'd be interested though to understand why you would want 2 VMs with the same Mac address - this would cause strange networking issues on the local LAN if the VMs are on the same segment

[marco201110142]

Interesting option but doesn't work.

Now the second VM is powered on without errors but cannot be reached via PING.

[kjb007]

you do understand that duplicate mac addresses will cause problems with communication, as you are seeing.  Assigning new IP addresses will not fix the problem if the underlying MAC address is the same.

Ethernet talks to MAC addresses, and it can't communicate to two different ones at the same time.

-KjB

[marco201110142]

I undersatd the problem.

I need to replicate some file servers that must have the same mac address to work properly. I noticed that using Virtualbox this is quite simple and works without particular problems on the network. However, Virtualbox VMs are not very stable in x64 OS environment so I was asking if VMWare is able to manage this situation. But maybe it is not possible...

[continuum]

this has been asked recently
then I recommended to use

ethernet0.checkMACAddress = "FALSE"
ethernet0.downWhenAddrMismatch = "false"
ethernet0.ignoreMACAddressConflict = "true"

and if I remember right it helped in that case

obviously this will only work if the 2 VMs do not live in the same network - as otherwise the OS of the VM will make sure that the second VM using the same MAC will not start the nic

[marco201110142]

Thank you for your suggestion but this is my case (2 VMs in the same network).

Unfortunately it seems Virtualbox can do something impossible for VMWare. Or maybe not?

[WoodyZ]

but this is my case (2 VMs in the same network).

Considering in the real word the likelihood of two physical network adapters on the same Subnet having the same MAC Address is highly unlikely nor would this be and acceptable condition, I therefore fail to see a reasonable explanation for why you would want to create that condition in a virtual network, and frankly IMO it's senseless!  Now, as an example, it's not senseless to replicate a given machine and maintain the same MAC Address to be used for testing purposes however it makes no sense to have them both up and running on the same Subnet at the same time.

[bulletprooffool]

The VMs coming up with the same MAC address questions has been resolved - you are now experiencing a normal networking problem. As stated before , matching MAC addresses on the same VLAN is going to cause problems. The nature of TCPIP is that a host will pick up and attempt to process on its local network based on the MAC address in the packet (unless the packet is broadcast) - IP addressing on a local level is used for routing decisions.

According to : http://support.microsoft.com/kb/120599

"The TCP/IP protocol may fail to start if another system using the same IP address is detected on the network. All the TCP/IP protocols currently shipping (as of 9/20/94) can detect duplicate IP addresses in most cases"

you have not explained a reason for creating duplicate Mac Addresses on your network? I suspect this is something related to a license that is bound to MAC Address? If this is the case, let the VMs have aut assigned MAC addresses and handle the 'OS' Mac address from within the VM.

Have a look at  : http://www.get-virtual.info/2011/02/24/292/

this has details on both statically assigning MAC addresses and managing MAC changes from within the OS (3rd party tools)

[marco201110142]

Bullettprooffo you are correct. Even if some needs may seem to be senseless, they can have sense in real world.

I’ve been asked by a customer to answer a specific problem. He has a custom client/server software solution bounded to his server (MAC Address) and, unfortunately, the software is rather old and no more under development. Now his old server is broken and he needs to replicate his old HW/SW configuration on a new server (here the need to assign a specific MAC Address).

Because of some limitations of the custom software (I don’t have details about it) the number of connections from clients to server is limited. But the customer needs to increase it (here the need to have 2 VMs with the same MAC Address but different IPs).

I’ve been only asked to suggest a technical solution to this problem (if exists).

My answer will be virtualization. So, I made some test running multiple VMs on the same network, with same MAC Address and different IPs. While using VMWare it seems not possible (or may it is possible but not easy), all seems to work fine with Virtualbox. However, the latter has some issues with x64 operating systems and, moreover, I believe VMWare is a more reliable, stable and mature product.

Obviously this is only a temporary solution to allow a new custom client/server software to be developed. But that’s my problem.

[bulletprooffool]

Okie - try OS level Mac Address changes . . whilst keeping the IP address presented to the VM on an auto assigned MAC.

You also have another alternative.

You could drop the 2 VMs with matching MACs onto different subnets (so MAC address conflits won't cause as many communication issues)

[marco201110142]

I have already set the MAC Address from inside the OS using the freeware Technitium MAC Address Changer v5 (http://www.technitium.com/tmac/index.html). But never solved.

[ShantanuVCP]

Having the same MAC address attached to multiple Virtual Machines is against the concept of Ethernet communication. As we understand that in a Ethernet based LAN, communication always happens with the help of MAC Address rather an IP address. Before any communication ARP protocol of the local host resolves IP address to MAC address of the destination host and stores it in the local cache; Once it is successful, then only it initiates communication with the destination host. This is the concept and all Ethernet communication abides by it. If there are two VM's with the same MAC Address, there will be significant packet drop in the network and communication would be almost unlikely.

So as per me this can't be a solution to any situation. This is against the law.

[marco201110142]

Is there anybody who can explain me why VirtualBox is able to manage multiple VMs with the same MAC Address (but different IP) and VMWare not (host: Windows 7 x64, guest: WIndows 7 x86)?

If it is not possible to have several VMs with the same MAC Address on the same network VirtualBox should not work!