Interlogix NS3500-28T-4S User Manual - Manual (Page 112)

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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 bits 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.
Adding an IEEE802.1Q Tag
Dest. Addr. Src. Addr. Length/E. type Data Old CRC
Dest. Addr. Src. Addr. E. type Tag Length/E. type Data New CRC
Priority CFI VLAN ID
■ Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one 802.1Q compliant
network device to another with the VLAN information intact. This allows 802.1Q VLAN to span network devices (and
indeed, the entire network – if all network devices are 802.1Q compliant).
Original Ethernet
New Tagged Packet