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
The first 2-bytes of the 801.1Q tag contains the HEX value 0x8100 which signals to the Ethernet receiver the presence of 802.1Q in the frame header.
The 802.1Q header also includes 3-bits as a priority field for applying COS markings and 12-bits for a VLAN ID field to mark the VLAN number the frame belongs to.
For original IEEE 802.3 Ethernet frames containing 802.2, a SNAP header is added with the OUI set to 00-00-00 and the Type set to 8100, this signifies a 802.1Q header follows the SNAP header.
802.1Q has the concept of a Native VLAN which is locally defined per switch and per port as the VLAN for which the traffic exiting the switchport will NOT contain a 802.1Q tag.
It is important to note that because the frame egressing on the Native VLAN does not contain a 1Q tag, it will also not contain any priority markings in the Ethernet header. While the packet my still contain DCSP/ToS priority markings in the IP header, most L2/L3 switches provide ingress QoS based on priority markings in the Ethernet header. Therefore it is not advisable to use the Native VLAN for sensitive traffic requiring QoS control. Some Ethernet switches allow you to tag the native vlan, but this configuration must be consistent on both ends of the link to avoid strange problems and lost traffic.