MAC address. Their presence is indicated by a value of 0x8100 in the Ether Type field. When a packet's
Ether Type field is equal to 0x8100, the packet carries the IEEE 802.1Q/802.1p tag. The tag is contained
in the following two octets and consists of 3 bits of user priority, 1 bit of Canonical Format Identifier (CFI -
used for encapsulating Token Ring packets so they can be carried across Ethernet backbones), and 12
bits of VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID is the VLAN identifier and is
used by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique VLAN can be identified.
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the
information originally contained in the packet is retained.
802.1Q Tag
User Priority
CFI
VLAN ID (VID)
3 bits
1 bits
12 bits
TPID (Tag Protocol Identifier)
TCI (Tag Control Information)
2
bytes 2
bytes
Preamble Destination
Address
Source
Address
VLAN TAG
Ethernet
Type
Data FCS
6 bytes
6 bytes
4 bytes
2 bytes
46-1517 bytes
4 bytes
The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether
Type/Length or Logical Link Control. Because the packet is now a bit longer than it was originally, the
Cyclic Redundancy Check (CRC) must be recalculated.
Adding an IEEE802.1Q Tag
Dest. Addr.
Src. Addr.
Length/E. type
Data
Old CRC
Dest. Addr.
Src. Addr.
E. type
Tag
Length/E. type
Data
New CRC
Priority CFI
VLAN
ID
New Tagged Packet
Original Ethernet
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q
compliant network device to another with the VLAN information intact. This allows 802.1Q VLAN to span
network devices (and indeed, the entire network – if all network devices are 802.1Q compliant).
Every physical port on a switch has a PVID. 802.1Q ports are also assigned a PVID, for use within the
switch. If no VLAN are defined on the switch, all ports are then assigned to a default VLAN with a PVID
equal to 1. Untagged packets are assigned the PVID of the port on which they were received. Forwarding