802.1Q Vlan Tags - D-Link DES-3326 User Manual

24-port NWay Ethernet Switch User's Guide

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 3 bits or 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 VLANs can be identified.
The tag is inserted into the packet header making the entire
packet longer by 4 octets. All of the information contained in the
packet originally is retained.
60 Switch Management and Operating Concepts