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 -
nd 12
y 802.1p. The VID is the VLAN identifier and is
sed by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique VLAN can be identified.
erted into the packet header making the entire packet longer by 4 octets. All of the
information originally contained in the packe
Prio ID)
used for encapsulating Token Ring packets so they can be carried across Ethernet backbones), a
bits of VLAN ID (VID). The 3 bits of user priority are used b
u
The tag is ins
t is retained.
802.1Q Tag
User rity CFI VLAN ID (V
3 bits 1 bits 12 bits
(Ta l Identif (Ta forma
TPID g Protoco ier) TCI g Control In tion)
2 bytes 2 s
Preamble Destination Source VLAN TAG Ethernet Data FCS
byte
Address Address Type
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 longe ginally, the
Cyclic Redundancy Check (CRC) must be recalculated.
ding an IEEE802.1Q Tag
Dest. Addr. Src. Addr. Length/E. type Data Old CRC
r than it was ori
Ad
Dest. Addr. Src. Addr. New CRC E. type Tag Length/E. type Data
Priority CFI VLAN ID
Port VLAN ID
Original Ethernet
New Tagged Packet
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).
port on which they were received. Forwarding
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
