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By Hugo Strydom, Managing Consultant, VMware Professional Sercices

In this first part of how LBT works we want to first explain how the stats are derived for the pNIC attached to the vDS (or vSwitch).

The vmkernel maintains tx/rx stats for each of the pNIC’s that is attached to a vDS. Stats to the vmkernel is updated as soon as the packet have been send or received by the pNIC. There is no overhead on delivering the packet to the VM during the stats update process to the vmkernel.

When a pNIC is not attached to a vDS, no stats will be collected for that pNIC. Have a look at ESXTOP and note that no stats is shown for pNIC’s that is not connected to a vSwitch/vDS. Also the in ESXTOP the pNIC is not shown if not connected to vSwitch/vDS.

ESX Host with 4 pNIC’s



Consider a vDS Port Group configured for LBT. When a VM is powered on the vNIC is attached to the DVS’s pNIC using “Route based on originating virtual port”(default when LBT is selected). At 30sec’s intervals the vmkernel will calculate the load on each pNIC that is attached to the vDS and if needed move the VM traffic over to another pNIC. Thus during VM startup the vNIC could be placed on a pNIC that is considered saturated but will be moved off to other pNIC’s that is not saturated once LBT have done a round of calculations to balance the load.

In part 2 we will explain how NLB calculate when to move VM traffic onto another pNic.