802.1Q
After Cisco created ISL, the IEEE completed work on the 802.1Q standard. 802.1Q uses a different style of header to tag frames with a VLAN number than the ISL. It does not encapsulate the original frame, but adds a 4-byte header to the original Ethernet header. This additional header includes a field with which to identify the VLAN number. Because the original header has been changed, 802.1Q encapsulation forces a recalculation of the original FCS field in the Ethernet trailer, because the FCS is based on the contents of the entire frame. 802.1Q also introduces the concept of a native VLAN on a trunk. Frames belonging to this VLAN are not encapsulated with tagging information. In the event that a host is connected to an 802.1Q trunk link, that host will be able to receive and understand only the native VLAN frames.