{"id":14091,"date":"2013-01-21T13:25:00","date_gmt":"2013-01-21T13:25:00","guid":{"rendered":"https:\/\/aidanfinn.com\/?p=14091"},"modified":"2013-01-21T13:25:00","modified_gmt":"2013-01-21T13:25:00","slug":"kb2804678cannot-exceed-256-dynamic-mac-addresses-by-default-on-hyper-v-host","status":"publish","type":"post","link":"https:\/\/aidanfinn.com\/?p=14091","title":{"rendered":"KB2804678\u2013Cannot Exceed 256 Dynamic MAC Addresses By Default On Hyper-V Host"},"content":{"rendered":"

This is not a scenario I\u2019ve encountered \u2026 not too many of us will ever have more than 256 virtual NICs\u00a0on <\/em>a single host.\u00a0 Microsoft has posted a support article<\/a> on this scenario:<\/p>\n

Windows Hyper-V server has a default limit of 256 dynamic MAC addresses.\u00a0 You have a Windows Server 2012 (WS2012) host which is configured with the Hyper-V role. The Hyper-V server is configured to provide dynamic MAC addresses to the guest machines:<\/p>\n

When you start a guest virtual machine, you may encounter the following error message:<\/p>\n

The application encountered an error while attempting to change the state of ‘<Virtual machine name>’<\/p>\n

Synthetic Ethernet Port (Instance ID CCE417C5-BDD9-4216-85CA-248620EE75C6): Failed to power on with Error ‘Attempt to access invalid address’.<\/p>\n

On a Windows Server 2008 or Windows Server 2008 R2 Hyper-V host, an Event ID 12565 from source “Microsoft-Windows-Hyper-V-Worker” is logged.<\/p><\/blockquote>\n

Here\u2019s how this issue is caused.\u00a0 Each host has a default dynamic pool of MAC addresses.\u00a0 This pool is generated as follows:<\/p>\n

    \n
  1. The first three octets of the MAC address pool are 00:15:5D.\u00a0 This is unique to all Microsoft \u201cdevices\u201d, complying with IEEE standards.<\/li>\n
  2. The next two octets (4 and 5) in the MAC address pool are derived from the IP address of the host (the last 2 octets of the IP address to be precise).\u00a0 This gives the pool some uniqueness in your server farm.\u00a0 We now have 5 of the 6 required octets for a MAC address.<\/li>\n
  3. Finally, the last octet in the MAC address pool is the range 0x0-0xFF.\u00a0 Each instance of this range is used once per virtual NIC (assuming that it\u2019s using a dynamic MAC) on this host<\/li>\n<\/ol>\n

    Octets 1 to 3 are unique to Microsoft \u201cdevices\u201d.\u00a0 Octets 4 and 5 are for giving the MAC address pool uniqueness for the pool range.\u00a0 And octet number 6 is used to make each dynamic MAC address unique on the network.<\/p>\n

    If you want to break out a scientific calculator or if you know your computer science, the clue to the cause is in that last piece of the puzzle.\u00a0 We only have octet 6 for each dynamic MAC address instance that can be allocated.\u00a0 An octet is 8 bits, from 00000000 to 11111111.\u00a0 That is 00 to FF in hex.\u00a0 Which is 0 to 255, or 256 numbers.\u00a0 And that means each host can have 256 MAC addresses, by default.<\/p>\n

    There are workarounds to this, if you are in the very rare<\/em> situation where you need more than 256 MAC addresses on a single host.<\/p>\n

    Use Static MAC Addresses<\/span><\/strong><\/p>\n

    Turn off (shut down) the VM and assign static MAC addresses.\u00a0 System Center VMM makes this easy with centrally human managed pool of MAC addresses, something like with DHCP.<\/p>\n

    Manually Modify The Per-Host Dynamic MAC Address Pool<\/span><\/strong><\/p>\n

    Be very careful with this!\u00a0 You don\u2019t want to create overlapping pools of MAC addresses to confuse ARP on your network.\u00a0 The MAC address range is defined by two registry values in HKLMSoftwareMicrosoftWindows NTCurrentVersionVirtualization:<\/p>\n