User’s Manual of GS-5220 LCD Series
188
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 VLAN 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.
802.1Q Tag
User Priority CFI VLAN ID (VID)
3 bits 1 bit 12 bits
TPID (Tag Protocol Identifier) TCI (Tag Control Information)
2 bytes 2 bytes
Preamble Destination
Address
Source
Address
VLAN TAG Ethernet Type Data FCS
6 bytes 6 bytes 4 bytes 2 bytes 46-1500 bytes 4 bytes
The Ether Type and VLAN ID are inserted after the MAC source address, but before the original Ether
Type/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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