NIC Selection with vSphere Load Balancing

A customer asked me how the load balancing algorithms worked in vSphere and how even the distribution was with 'Route based in the originating virtual port ID’ which is the default policy. Note that the initial placement policy is also the same for the LBT (Load Based Teaming) policy.


I knew that we could check which pNIC a VM had been assigned to using ESXTOP and pressing n to look at the network info. (Unfortunately my lab where I took this screenshot only has a single NIC)

I went searching for the algorithm that is used to determine which pNIC is selected when a VM powers on and found the following:

Algorithm: (virtualPortID>>1) % numActiveNICs

Or in my basic understanding of maths notation:

pNIC selection = (virtualPortID / 2) modulo numActiveNICs

(modulo =  Given two positive numbers, a (the dividend) and n (the divisor), a modulo can be thought of as the remainder, on division of a by n.)

As an example: A NIC team has vmnic3, vmnic4, vmnic5 (active in this order).

Therefore vmnic3 = 0, vmnic4 = 1, vmnic5 = 2

So if we look at the port-ids on a vSwitch we can use this to work out which pNIC the VM will be assigned to.

/net/portsets/<vswitch>/ports/> ls

    67108865/

    67108873/ ===> 67108873 /2 = 33554436 modulo 3 = 0 (VM will use vmnic3)

    67108874/ ===> 67108874 /2 = 33554437 modulo 3 = 1 (VM will use vmnic4)