D-Link DGS-3324SR - Page 63

D-Link DGS-3324SR Layer 3 Stackable Gigabit Ethernet Switch
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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- 19. 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.
Figure 4- 20. Adding an IEEE 802.1Q Tag