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Ondrej Sevecek's English Pages > Posts > Microsoft NLB is incompatible with SR-IOV hardware acceleration on Hyper-V
February 28
Microsoft NLB is incompatible with SR-IOV hardware acceleration on Hyper-V

If you plan to use Microsoft NLB (Network Load Balancing) in unicast mode in Hyper-V virtual machines, these VMs must not use any Hyper-V virtual switch configured for SR-IOV (single root IO virtualization). Otherwise the virtual machine does not receive any traffic on the virtual NLB MAC address.

The unicast mode of NLB uses a virtual unicast MAC address. The problem with SR-IOV though is that it needs the virtual machines to register all their unicast MAC addresses with the physical network adapter in order to have the hardware acceleration.

The SR-IOV is a technology supported by server hardware NIC (network interface cards) which can distribute network traffic directly to the virtual machines (VMs) according to either their destination MAC address or their assigned VLAN ID. In order to tell the SR-IOV NIC the actual MAC address of a VM the hosting Hyper-V must know the MAC address of the virtual machine. In case of NLB the virtual MAC address is managed dynamically by the NLB driver inside the VM and the hypervizor does not know anything about it.

This function depends on the virtual switch being configured as non-SR-IOV. It does not matter if you enable the SR-IOV in the properties of the VM's network port. You must not have the SR-IOV enabled on the external virtual switch.


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