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Event ID: 28 Source: VMSMP

Port "943008F1-E4EA-498A-891E-0BB80BDE935F" was prevented from using MAC address "00-C0-DD-11-68-51" because it is pinned to port "04769FD6-DA39-4467-910C-44C2F0DA1B12".
I also had this problem and it ended up to have nothing to do with the server, but was a looped cable on the network. I would suggest that if your server was working properly for a period of time before this message showed up, that you check for bad cables or other hardware before you reconfigure your server.
From a Microsoft support forum:
The event log fills with event 28 saying the MAC address is blocked because it assigned to SCVMM. After some investigation I found a tutorial about dynamic MAC addresses in Hyper-V on a Microsoft blog:
EV100547 (Hyper-V: MAC Address allocation and apparent network issues MAC collisions can cause).

In my case the problem was caused because I had done a P2V conversion, switched off the Physical machine and left it off but then a few weeks later setup the physical machine as another server.

What had happened is that the P2V had taken the MAC address from the physical server and applied it to the virtual machine (there lies the problem), then when I reconfigured the physical machine and added it to the network I started getting network problems.

Solution: Use a dynamically assigned MAC address from the pool Hyper-V allocates for VMs by changing the properties of the VM before you boot it up.

This problem can also happen in Microsoft Virtual Server, it is not related just to Hyper-V.
I had a similar problem with HP NCU network teaming and Hyper-V. The solution was a software update. See EV100297 (Hyper-V , HP network teaming and VMSMP Event ID 28).
From a support forum: I followed the suggestions from EV100143 (Hyper-V: MAC Address allocation and apparent network issues MAC collisions can cause) and the problem was fixed (I changed the MAC address and now connectivity is restored).

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