Let's say that you have 25 Gbps SFP+ Network card in your host and have created a Windows Virtual Machine with a 1 Gbps Nic card and now confused regarding the bandwidth for the virtual machine and how the Virtual Nic of the VM is different than the Host Nic when it comes to bandwidth and functionality.
vNic is the software NIC emulation in the VM and is just a driver and in latest VMs the NIC is para-virtualised, which means that the NIC and Operating System is aware of the Host Physical NIC capability and understands that the vNic is just a driver.
So what basically is this driver supposed to do?
The vNic driver is really an API between the guest and the hypervisor so the vNic bandwidth is totally disconnected from any physical hardware.
So what is the Physical NIC card then?
Physical Nic is the physical network adapter connected to your physical switch and is responsible for the transfer of packets in your environment.
So how Para virtualised vNic is better, does it have any advantages?
Of course, it is advantageous as compared to legacy adapters.
VM-VM communication inside a host is not dependent on the host bandwidth nor the VM capability. The VM just dumps frames onto the host and it is the host job to deal with the physical devices and no need for the VM to know or care what the link speed of hosts physical devices are.
One another advantages of this are that when packets are moving between 2 VMs on the same host they can send packets as fast as the host's CPU can move them from one set of memory to another.
This also allows the host to do adaptor teaming and spread traffic across multiple links without the need for the VM to be explicitly configured to get the full bandwidth of the setup.
To summarise the information we gathered till now, we can safely say that even if you have configured your VM with a 1 Gbps NIC card, the vNIC is para-virtualised and is aware that it's a piece of code and will dump all the packets to the Host Hardware irrespective of the configured bandwidth at VM level.
To measure the bandwidth, we can always try iperf utility which is an industry-standard to benchmark the bandwidth performance.
(Please note to follow the recommended guidelines for the iperf utility before testing the bandwidth.)
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AHV Networking and best practises.