Virtual NIC Link Speed - is it really speed?

This will be a quick blog post, prompted by another question I received about VMware virtual NIC link speed. In this blog post I’d like to demonstrate that the virtual link speed shown in operating systems is merely a reported value and not an actual limit on throughput.

I have two Linux Mint (Debian based) systems mlin01 and mlin02 virtualized in VMware vSphere 8.0.3. Each system has VMXNET3 NIC. Both virtual machines are hosted on the same ESXi host, so they are not constraint by physical network. Let's test network bandwidth between these two systems with iperf.

I run following command on mlin02 to have iperf server up, running, and listening

iperf -s

Then I run client command on mlin01 to generate network traffic to iperf server.

iperf -c mlin02 -P4 -i5 -t60 

Here is the screenshot from the test

iperf test to show that reported link speed is not the network throughput limit

As shown in the screenshot above, the VMXNET3 network interface reports a speed of 10,000 Mb/s (10 Gb/s), yet iperf demonstrates a network throughput of 16.6 Gb/s which exceeding the reported link speed.

I hope this clearly demonstrates that the VMXNET3 network interface is a virtualized device, and as such, it's not constrained by the physical limitations we typically encounter with hardware. By the way, you can observe the same behavior on E1000 network interface.