DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet Switch User Manual
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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 fie ld. 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 t wo 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). T he 3 bits of us er priority are us ed
by 802.1p. The VID is the VLAN identifier and is used by the 802.1Q standard. B ecause th e VID is 12 bi ts long, 4094
unique VLANs can be identified.
The tag is inserted into the packet header making the entire packet longer b y 4 octets . All of the inform ation origina lly
contained in the packet is retained.
Figure 3 - 3 IEEE 802.1Q Tag
The EtherType and VLAN ID are inserted after the MAC source addres s, but bef ore the ori ginal EtherT ype/Length or
Logical Link Control. Because the packet is now a bit longer than it was originally, the Cyclic Red undancy Check
(CRC) must be recalculated.
Figure 3 - 4 Adding an IEEE 802.1Q Tag