802.1Q Vlan Tags - D-Link DES-6500 User Manual

D-Link DES-6500 Layer 3 Stackable Gigabit Ethernet Switch

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 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 VLANs 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.
Figure 4- 16. IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source address, but before the
original EtherType/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.
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