32-bit SNMP interface counters (ifInOctets/ifOutOctets) are sourced by hardware statistics retrieved from interface registers.
These counters are valid for MPX/SDX, loopback interface and legacy-emulated(E1000) interfaces in ESX-VPX. But are not valid for VMXNet3 interfaces.
Since hardware statistics retrieval was not supported for PV drivers (VMXNet3), these counters will show Zero for PV interfaces (VMXNet3).
As per RFC2233, High capacity octet counters (expanded 64-bit counters) were adapted for high capacity interfaces operating at speeds higher than 20 million bits per second, in which 32-bit counters do not provide enough capacity and wrap too fast.
“For interfaces that operate at 20,000,000 (20 million) bits per
second or less, 32-bit byte and packet counters MUST be used. For
interfaces that operate faster than 20,000,000 bits/second, and
slower than 650,000,000 bits/second, 32-bit packet counters MUST
be used and 64-bit octet counters MUST be used. For interfaces
that operate at 650,000,000 bits/second or faster, 64-bit packet
counters AND 64-bit octet counters MUST be used.”