A VLAN can span across multiple switches, or even routers. This enables hosts in a VLAN to be
dispersed in a looser way. That is, hosts in a VLAN can belong to different physical network
segment. This switch supports three ways, namely, 802.1Q VLAN, MAC VLAN and Protocol
VLAN, to classify VLANs. VLAN tags in the packets are necessary for the switch to identify
packets of different VLANs. The switch can analyze the received untagged packets on the port
and match the packets with the MAC VLAN, Protocol VLAN and 802.1Q VLAN in turn. If a
packet is matched, the switch will add a corresponding VLAN tag to it and forward it in the
corresponding VLAN.
7.1
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 7-2 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.
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