Total Pageviews

My YouTube Channel

Tuesday, 25 June 2013

Sample Configuration - Network Load Balancing (NLB) unicast mode configuration (1006778)


Requirements and notes for setting up NLB unicast mode:
  • Two machines running Windows Server 2003 or later
  • Each machine should have at least two network cards and at least one fixed IP address
  • VMware recommends two adapters in each machine for best performance
  • One adapter mapped to the real IP address (Microsoft calls this the Dedicated IP), and one mapped to the virtual IP address (Microsoft calls this the Cluster IP)
  • A second network card is required for communication between the servers
  • A benefit of unicast mode is that it works out of the box with all routers and switches (since each network card only has one MAC address)
  • In unicast mode, since all hosts in the cluster have the same MAC and IP address, they do not have the ability to communicate with each other via their NLB network card


To configure NLB unicast mode:

  1. NLB unicast mode requires two NICs.
  2. Add two virtual NICs to NLB-configured virtual machines.
  3. Use the Add New Hardware wizard to add new hardware to the virtual machine. The virtual hardware that you add appears in the hardware list displayed in the Virtual Machine Properties wizard. The selected guest operating system determines the devices that are available to be added to a given virtual machine.
  4. Add an Ethernet adapter (NIC):

    1. Start the Add Hardware wizard.
    2. Click Ethernet Adapter, and click Next.
    3. In the Network connection panel, choose either a named network with a specified label or a legacy network.
    4. To connect the virtual NIC when the virtual machine is powered on, select Connect at power on.
    5. To complete the wizard, click Finish.
  • ESXi/ESX vSwitch properties Notify Switches = NO
  • Unicast mode reassigns the station (MAC) address of the network adapter for which it is enabled and all cluster hosts are assigned the same MAC (media access control) address, you cannot have ESXi/ESX send ARP or RARP to update the physical switch port with the actual MAC address of the NICs as this break the the unicast NLB communication
  • ESXi/ESX vSwitch security properties must be set to default, Forged Transmits = Accept.
  • VMware recommends configuring the cluster to use NLB multicast mode even though NLB unicast mode should function correctly if you complete these steps. This recommendation is based on the possibility that the settings described in these steps might affect vMotion operations on virtual machines. Also, unicast mode forces the physical switches on the LAN to broadcast all NLB cluster traffic to every machine on the LAN. For more information on the differences, seeMicrosoft Network Load Balancing Multicast and Unicast operation modes (1006580). If you plan to use NLB unicast mode, note that:

    • All members of the NLB cluster must be running on the same ESXi/ESX host.
    • All members of the NLB cluster must be connected to the single portgroup on the virtual switch
    • vMotion for unicast NLB virtual machines is not supported (unless you want to migrate all NLB members to a different ESXi/ESX host)
  • For troubleshooting information see: Microsoft NLB not working properly in Unicast Mode (1556).
  • Unicast NLB is not supported on the Cisco Nexus 1000V Switches, except version 4.2(1) SV1(5.1) and later.

    Note: The preceding link was correct as of December 4, 2012. If you find the link is broken, provide feedback and a VMware employee will update the link.

Example screen shots:

Additional Information

Weak and strong host behavior in Windows

Windows XP and Windows Server 2003 use the weak host model for sends and receives for all IPv4 interfaces, and the strong host model for sends and receives for all IPv6 interfaces. You cannot configure this behavior.

The Next Generation TCP/IP stack in Windows Vista and Windows Server 2008 supports strong host sends and receives for both IPv4 and IPv6 by default on all interfaces, except the Teredo tunneling interface for a Teredo host-specific relay. For more information, see Strong and Weak Host Models on the Microsoft TechNet site.

Note: The preceding link was correct as of December 4, 2012. If you find the link is broken, provide feedback and a VMware employee will update the link.

To change this behavior and revert to the weak host model (which was used in Server 2003), open a Windows command prompt and run these commands:

netsh interface ipv4 set interface "Local Area Connection" weakhostreceive=enable
netsh interface ipv4 set interface "Local Area Connection" weakhostsend=enable

To verify that the changes are in effect, run this command:

netsh interface ipv4 show interface "Local Area Connection"

Where Local Area Connection is the name of the network interface obtained from running ipconfig /all from a Windows command prompt.