One importanty thing to know when administering VMWare ESX servers is that the virtual switches in ESX Server do support VLAN (IEEE 802.1Q) trunking (also called VLAN tagging). Using VLANs, VMWARE admins can leverage their network infrastructures with ESX Server.
I have just found this interesting white paper which provides an overview of VLAN concepts and benefits and illustrates three possible ESX Server and virtual machine VLAN configurations.
Example of VLAN tag (802.1Q) in Ethernet frame:
As you can see, 802.1Q does not actually encapsulate the original frame. Instead, it adds a 32-bit field between the source MAC address and the Type/Length fields of the original frame.
For knowing more, just refer to: