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After 3 years of running perfectly with Hyper-V I am all of a sudden having network problems with Hyper-V on win 2012 R2. I set up my servers 3 years ago and never had these problem until this week.

About the time I started having problems I started for the first time seeing these events in the logs. Event IDs 16945 in the System logs. "MAC conflict: A port on the virtual switch has the same MAC as one of the underlying team members on Team Nic Microsoft Network Adapter Multiplexor Driver."

This command shows I have the same MAC address for the first three entries.

get-netadapter | sort macaddress

pNIC2

tNIC1

vNIC1

pNIC1

pNIC1 and pNIC2 are the physical network adapters tNIC1 is the the NIC team of the two physical NICs pNIC1 and pNIC2 in Switch Independent, Dynamic load balancing, standby adapters none. vNIC1 is the virtual NIC of the server.

The servers have one Virtual switch, vSwitch1, connection type External network - Microsoft Network Adapter Driver and Allow management OS to share network adapter. No vlans are used and the only extensions checked is the MS filtering platform.

I have 4 identical servers hardware and software set up this way. Running get-netadapter | sort macaddress on each of the 4 servers shows the exact same thing. The first three adapters have the same MAC address on all 4 servers but the MAC address on the different servers are all different. In other words there are no server to server MAC address conflicts.

Server 1
pNIC1    Intel(R) 82576 Gigabit Dual Port Net...      12 Up           00-25-90-2D-68-1A         1 Gbps
tNIC1    Microsoft Network Adapter Multiplexo...      16 Up           00-25-90-2D-68-1A         2 Gbps
vNIC1    Hyper-V Virtual Ethernet Adapter #2          19 Up           00-25-90-2D-68-1A        10 Gbps
pNIC2    Intel(R) 82576 Gigabit Dual Port N...#2      13 Up           00-25-90-2D-68-1B         1 Gbps

Server 2
pNIC1    Intel(R) 82576 Gigabit Dual Port Net...      12 Up           00-25-90-2C-6A-64         1 Gbps
tNIC1    Microsoft Network Adapter Multiplexo...      16 Up           00-25-90-2C-6A-64         2 Gbps
vNIC1    Hyper-V Virtual Ethernet Adapter #2          19 Up           00-25-90-2C-6A-64        10 Gbps
pNIC2    Intel(R) 82576 Gigabit Dual Port N...#2      13 Up           00-25-90-2C-6A-65         1 Gbps

Server 3
Name     InterfaceDescription                    ifIndex Status       MacAddress             LinkSpeed
----     --------------------                    ------- ------       ----------             ---------
pNIC2    Intel(R) 82576 Gigabit Dual Port N...#2      13 Up           00-25-90-4D-65-2A         1 Gbps
tNIC1    Microsoft Network Adapter Multiplexo...      16 Up           00-25-90-4D-65-2A         2 Gbps
vNIC1    Hyper-V Virtual Ethernet Adapter #2          19 Up           00-25-90-4D-65-2A        10 Gbps
pNIC1    Intel(R) 82576 Gigabit Dual Port Net...      12 Up           00-25-90-4D-65-2B         1 Gbps

Server 4
Name     InterfaceDescription                    ifIndex Status       MacAddress             LinkSpeed
----     --------------------                    ------- ------       ----------             ---------
pNIC1    Intel(R) 82576 Gigabit Dual Port Net...      12 Up           00-25-90-4D-40-E6         1 Gbps
tNIC1    Microsoft Network Adapter Multiplexo...      16 Up           00-25-90-4D-40-E6         2 Gbps
vNIC1    Hyper-V Virtual Ethernet Adapter #2          19 Up           00-25-90-4D-40-E6        10 Gbps
pNIC2    Intel(R) 82576 Gigabit Dual Port N...#2      13 Up           00-25-90-4D-40-E7         1 Gbps

I read where it is common and OK to see the same MAC address on one of the vNICs and one of the pNICs,
but that it isn't OK for a pNIC to have the same address as the NIC team tNIC1.
https://serverfault.com/questions/702570/server-2012-r2-hyper-v-virtual-switch-host-mac-address-conflict-331flr

What do I need to do to resolve the conflicts and keep them from happening?

I researched how to set these MAC address's manually and know how to do that including making sure I use MAC address's outside of the dynamic range set on the vSwitch, but I would like to get some feed on this before making those changes and wondering why after all of these years this just started happening?

Thank you!

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