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802.1Q Packet Forwarding
802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the
source MAC address. Their presence is indicated by a value of 0x8100 in the EtherType field. When
a packet
’
s EtherType 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 three bits or user priority, one bit of Canonical
Format Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across
Ethernet backbones) and twelve bits of VLAN ID (VID). The three 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
twelve bits long, 4094 unique VLANs can be identified.
The tag is inserted into the packet header making the entire packet longer by four octets. All of the
information contained in the packet originally is retained.
