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I am trying to troubleshoot a performance issue with our application on a customer network. We do not support the network infrastructure, and the people that do have effectively washed their hands of the problem.

On examining the System log on the Windows Server 2008 R2 host I see that there are thousands of event id 28 logged for VMSMP - these are MAC address collision warnings. When I say multiple, I mean 40 or 50 per second.

So - this is obviously not good, but is it bad enough to adversely affect network performance?

HopelessN00b
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Alan B
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1 Answers1

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Yes. This would definitely have negative effects on network performance. Switches use MAC addresses to learn which switch port to send traffic to. If the switch doesn't know what port to send traffic out on, it will send the frames out to all ports, causing a flooding situation. You'd likely also experience intermittent connectivity on the hosts with duplicate MACs, as the switch sends some traffic to one host, and some traffic to the other.

You really have to fix this. You don't want duplicate MAC addresses in the same collision domain.

Edit: VMSMP Event ID 28 appears to not be about MAC address collisions actually happening on the network though, but rather Hyper-V preventing the traffic from occurring in the first place because of a duplicate MAC address in the configuration.

Port 'SWITCHPORT-SM-A6285CC8-5521-4180-BEE9-59C9929D26CB-1-1' was prevented from using MAC address '00-15-5D-64-3A-16' because it is pinned to port '27263E05-4CB3-4751-9'

I don't know if you're using NIC teaming on your Hyper-V hosts, and dynamic MACs for your VMs, but consider sorting things out by assigning static MACs that don't conflict.

http://blogs.technet.com/b/jhoward/archive/2008/07/15/hyper-v-mac-address-allocation-and-apparent-network-issues-mac-collisions-can-cause.aspx

Ryan Ries
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  • many thanks Ryan - I just needed someone with smarts in this area to tell me this so I can start to lean on them! Yes, there appears to be teaming set up and MAC assignment appears to be dynamic. Will get it changed. – Alan B Jun 05 '13 at 15:59
  • I am having this same problem however my network and HyperV server is quite simple. I have one HyperV Server 2008 R2 Standard with three guest OSs. I've followed the advice about hardcoding the MAC addresses for the guest OSs and set them to 00-15-5D-01-01-01, 02 & 03 just to ensure they weren't in the 00-xx space. I'm still getting this problem on the new MAC Address. – Tony Toews Mar 18 '15 at 04:04