D-Link DGS-3308FG Product Manual - Page 35

Q VLAN Tags

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8-port Gigabit Ethernet Switch User's Guide Figure 5-3. IEEE 802.1Q Packet Forwarding 802.1Q VLAN Tags 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 field. 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 two octets and consists of 3 bits or 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. 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 5-4. IEEE 802.1Q Tag 25

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8-port Gigabit Ethernet Switch User’s Guide
25
Figure 5-3.
IEEE 802.1Q Packet Forwarding
802.1Q VLAN Tags
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 field. 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 two octets and consists of 3
bits or 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.
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 5-4.
IEEE 802.1Q Tag