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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 75
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.
Figure 5-7. 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.