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6.1 802.1Q VLAN
VLAN tags in the packets are necessary for the switch to identify packets of different VLANs. The
switch works at the data link layer in OSI model and it can identify the data link layer encapsulation
of the packet only, so you can add the VLAN tag field into the data link layer encapsulation for
identification.
In 1999, IEEE issues the IEEE 802.1Q protocol to standardize VLAN implementation, defining the
structure of VLAN-tagged packets. IEEE 802.1Q protocol defines that a 4-byte VLAN tag is
encapsulated after the destination MAC address and source MAC address to show the information
about VLAN.
As shown in the following figure, a VLAN tag contains four fields, including TPID (Tag Protocol
Identifier), Priority, CFI (Canonical Format Indicator), and VLAN ID.
Figure 6-2 Format of VLAN Tag
（1） TPID: TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. By default, it is
0x8100.
（2） Priority: Priority is a 3-bit field, referring to 802.1p priority. Refer to section “QoS & QoS
profile” for details.
（3） CFI: CFI is a 1-bit field, indicating whether the MAC address is encapsulated in the
standard format in different transmission media. This field is not described in detail in this
chapter.
（4） VLAN ID: VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packet
belongs. It is in the range of 0 to 4,095. Generally, 0 and 4,095 is not used, so the field is in
the range of 1 to 4,094.
VLAN ID identifies the VLAN to which a packet belongs. When the switch receives an
un-VLAN-tagged packet, it will encapsulate a VLAN tag with the default VLAN ID of the inbound
port for the packet, and the packet will be assigned to the default VLAN of the inbound port for
transmission.
In this User Guide, the tagged packet refers to the packet with VLAN tag whereas the untagged
packet refers to the packet without VLAN tag, and the priority-tagged packet refers to the packet
with VLAN tag whose VLAN ID is 0.
Link Types of ports
When creating the 802.1Q VLAN, you should set the link type for the port according to its
connected device. The link types of port including the following three types:
（1） ACCESS: The ACCESS port can be added in a single VLAN, and the egress rule of the
port is UNTAG. The PVID is same as the current VLAN ID. If the ACCESS port is added to
another VLAN, it will be removed from the current VLAN automatically.
（2） TRUNK: The TRUNK port can be added in multiple VLANs, and the egress rule of the port
is TAG. The TRUNK port is generally used to connect the
cascaded network devices for it
can receive and forward the packets of multiple VLANs. When the packets are forwarded
by the TRUNK port, its VLAN tag will not be changed.
（3） GENERAL: The GENERAL port can be added in multiple VLANs and set various egress
rules according to the different VLANs. The default egress rule is UNTAG. The PVID can
be set as the VID number of any VLAN the port belongs to.
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