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Can this situation lead to a conflict of MAC addresses?

I used before Windows 8.1 and Server 2012 R2 to setup VM environments several times already, but now I'm doing it by hand (meaning Powershell), because it's a Hyper-V Server 2012R2 (the free alternative). Normally, when a virtual switch is created from Hyper-V Manager GUI, it gets one of these Microsoft-owned MAC addresses (00-15-5D-XX-XX-XX).

Name                      InterfaceDescription                    ifIndex Status       MacAddress             LinkSpeed
----                      --------------------                    ------- ------       ----------             ---------
Ethernet 2                Intel(R) 82579LM Gigabit Network Con...      13 Up           F8-B1-56-D0-5C-D3         1 Gbps
vEthernet (SwitchZew_I... Hyper-V Virtual Ethernet Adapter #2          17 Up           00-15-5D-00-E7-00        10 Gbps

Now, on this new Hyper-V host, I used a PS cmdlet New-VMSwitch, which didn't have any option to set it's MAC address, and the switch was created with exactly the same MAC as physical NIC. So when I type get-netadapter, I get:

Name                      InterfaceDescription                    ifIndex Status       MacAddress             LinkSpeed
----                      --------------------                    ------- ------       ----------             ---------
Ethernet 2                Intel(R) 82579LM Gigabit Network Con...      13 Up           D4-C9-EF-F2-5F-79         1 Gbps
vEthernet (SwitchZew)     Hyper-V Virtual Ethernet Adapter #2          18 Up           D4-C9-EF-F2-5F-79        10 Gbps

Two MACs on the same network worry me. Can this be a problem?

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If these MAC addresses were on different segments of the network then it could lead to an arp spoofing type situation, where a switch had to decide which path to send a particular packet down.

However because they are ultimately attached to the same interface, the network card will receive the packet and then pass it upwards to the Hyper-v switch. (particularly because the interface will be in promiscuous mode at that point)

Unless you are seeing a specific issue that could be related to this, it isn't something I would be concerned with, and would imagine that it is actually done like this by design.

  • Right, so is it true then that: 1) promiscuous mode is ON by default, when virtual switch is created (wikipedia says so), so hypervisor can decide whether to forward that packet to a virtual interface it hosts. 2) And it would just forward to host OS when virtual switch and physical machine have same MAC address? – Kitet Aug 21 '15 at 5:21
  • @Kitet an adapter in non promiscuous mode would filter all traffic out not specifically addressed to it (unless it is broadcast) a promiscuous mode adapter passes every packet up to the kernel to decide what to do with. I don't know the architecture but I imagine that Hyper-v places a filter in the network stack that intercepts all traffic on the way to the kernel. – Michael B Aug 21 '15 at 7:27

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