IEEE 802.1Q (sometimes referred to as 1Q or DOT1Q) is a industry standards based implementation of carring traffic for multiple VLANs on a single trunking interface between two Ethernet switches. 802.1Q is for Ethernet networks only.
Unlike ISL , 802.1Q does not encapsulate the original Ethernet frame.
For Ethernet V2 frames, 802.1Q inserts a new 4-byte field between the SA and TYPE fields of the original Ethernet frame. Since the addition of this new DOT1Q field changes the original frame, 802.1Q also recomputes and rewrites the original 4-byte FCS at the end of the frame.
802.1Q Header Insertion into Ethernet II frame