ESX Server requires these ID tags to route traffic in the two VLAN modes supported within the system:
You should avoid using a native VLAN when designing the configuration for your external and virtual switches. Using specific ID tags, rather than the default VLAN ID, may prevent networking failures if an external switch is returned to its default settings. Also, avoiding native VLANs provides a more secure interface between virtual machines and your network.
If you are already using a native VLAN, you can reconfigure your switch to pass through ID tags on all ports, including native traffic (that is, messages using the ID of the native VLAN). Set the switch to enable 802.1q tags for all VLAN ID values, either globally or for the trunk ports connected to your system. See the documentation for your switch for details.
For more information about VLANs and ESXi/ESX Servers, see the VLAN Configuration section in the vSphere Networking guide.