One of the enhancements in ESX 3.5 Update 4 is the support for vmxnet. The support for this adapter is a lot better since Update 4 is released. It is possible now that you can add the Enhanced vmxnet adapter to a Windows Server 2003 VM without goint through a workaround described in http://kb.vmware.com/kb/1007195. You can use the VI client to change adapter type.

enhanced-vmxnet.jpg

Enhanced vmxnet - The enhanced vmxnet adapter is based on the vmxnet adapter but provides some high-performance features commonly used on modern networks, such as jumbo frames. This virtual network adapter is the current state-of-the-art device in virtual network adapter performance, but it is available only for some guest operating systems on ESX Server 3.5.
You don’t need to go through http://kb.vmware.com/kb/1007195 any more.

Enhanced VMXNET is supported only for a limited set of guest operating systems:

  • 32/64-bit versions of Microsoft Windows 2003 (Enterprise and Datacenter Editions). You can use enhanced vmxnet adapters with other versions of the Microsoft Windows 2003 operating system, but a workaround is required to enable the option in the VI Client. See

  • 32/64-bit versions Red Hat Enterprise Linux 5.0

  • 32/64-bit versions SUSE Linux Enterprise Server 10

  • 64-bit versions Red Hat Enterprise Linux 4.0

And some issues:

Migrating virtual machines that use enhanced vmxnet. Enhanced vmxnet is new with ESX Server 3.5. Virtual machines configured to have enhanced vmxnet adapters cannot migrate to older ESX Server hosts, even though virtual machines can usually migrate freely between ESX Server 3.0 and ESX Server 3.0.1.
If you must migrate a virtual machine between newer and older hosts,don’t choose enhanced vmxnet but instead one of the older adapter types. Flexible or e1000 are offered whenever enhanced vmxnet is offered.

Upgrading from ESX Server 2.x to ESX Server 3.x. When a virtual hardware upgrade operation transforms a virtual machine created on an ESX Server 2.x host to an ESX Server 3.x host, Vlance adapters are automatically upgraded to Flexible. In contrast, vmxnet adapters are not upgraded automatically because certain guest operating systems - specifically most or all Linux versions - do not reliably preserve network settings when a network adapter is replaced.

Because the guest operating system thinks a Flexible adapter is still Vlance, it retains the settings in that case. If the upgrade were to replace a vmxnet adapter with a Flexible adapter, the guest operating system would erroneously discard the settings.

After the virtual hardware upgrade, the network adapter is still vmxnet, without the fallback compatibility of the Flexible adapter. Just as on the original older host, if VMware Tools is uninstalled on the virtual machine, it is unable to access its network adapters.

http://www.vladan.fr/enhanced-vmxnet-in-esx-35-update-4/