Re: How do I set up ESXi 5.1 and FreeBSD to use VMXNET 3?

What version of FreeBSD?  Check the compatibility matrix to ensure it's fully supported by ESXi 5.1, including VMXNET3:

http://www.vmware.com/resources/compatibility/vcl/result.php?search=FreeBSD&searchCategory=Guest+OS

For example, FreeBSD 7.4 does not support VMXNET3 under ESXi 5.1:

http://partnerweb.vmware.com/comp_guide2/detail.php?deviceCategory=software&productid=17579&supRel=76,168,171,187&vcl=true&testConfig=16

In contrast, FreeBSDE 9.0 supports VMXNET3 under ESXi 5.1:

http://partnerweb.vmware.com/comp_guide2/detail.php?deviceCategory=software&productid=17596&supRel=171,218&vcl=true&testConfig=16

Also, what version is the virtual hardware running this VM?  VMXNET3 is available in v7 and later:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1001805