Everything inside of the ESX enviroment is virtual, including the virtual switches that connect to the physical network, to think about virtualised switches is quite new for most people, but Cisco's high end routers have done this for a while !
Because ESX is enterprise level, the network components are also , the uplink of an ESX virtual switch is linked to one or more physical NIC's in the host machine, in a team or load balanced arrangment, or as seperate non redundant ports.
Some network expertise might be required outside of ESX also, as the ESX hosts connect from the NIC to the physical switches,ESX and the physical switches have to be configured to support some common form of VLAN tagging in most enterprises
Because of limitations within ESX, you can only have upto 6 PCI interfaces in a virtual machine, if you wanted to connect to 4 VLAN's in your network from a virtual machine you would be already fill 4 PCI slots, and the harddisk is also 1 PCI device, like wise the standard VGA console adapter takes a PCI slow too. This means we are already meeting the limit and cannot connect to further VLAN's
A solution to this is to get both ESX and the physical network configured for 802.1q trunking allowing a single virtual network device to access the permitted range of VLAN's specified in the physical switch side, some expertise may be required to test and implement this on the physical network side, it can be tricky as there are many variations and differences in switch confgurations and manufacturers, ideally your expertise comes from your in house network team who know the kit well ..
This need of network expertise is not really imposed by ESX itself, it is merely the nature of the network ESX connects to, on the ESX side the configuration is relativley straight forward and simple, most problems with 802.1q trunking will turn out to be a VMWare ESX config issue or a physical switch configuration issue, just because virtual switch configuration is really not too exhaustive and it is difficult to make a mess of it.
