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Second network adapter and TCP Offload

08 Nov

I have been having all fun and games trying to get a second NIC to work correctly using Microsoft Hyper V (core and full install). I could see the NIC and configure but the VM would not see the network.

I first thought that Hyper V didn’t like the brand of network card, so I ordered some others. But in the meantime, I have found a solution.

It comes down to an advanced NIC option – TCP Offload. This option is normally on by default on the NIC’s that I have seen. The TCP Offload is an option which was first used way-back-when PC’s were not as powerful as they are now. It would give the CPU a break from digesting network traffic and send it straight to the NIC. In this modern era, we do not need to do this any more. Also, disabling this option is supposed to assist in quicker network transfer speeds – we will see.

So how do you disable it? Open up your network card properties and click on the ADVANCED tab. The screen will look similar to this:

The 3 options that I disable are “IPv4 Checksum Offload”, “TCP Checksum Offload (IPv4)” and “TCP Checksum Offload (IPv6)”. Please note, the screen shot above has lumped in TCP and UDP together – that’s OK. Your network card driver may do a similar thing or separate them. Just disable TCP when you can.

Your VM’s will work like magic now.

 

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