TP-Link TL-SG2008 manual 802.1Q Vlan, ¾ Link Types of ports

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 in this switch.

（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.

¾Link 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 two types: Untagged and Tagged.

（1） Untagged: The untagged port can be added in multiple VLANs. If a VLAN-tagged packet arrives at a port and the VLAN ID in its VLAN tag does not match any of the VLAN the ingress port belongs to, this packet will be dropped. The packets forwarded by the untagged port are untagged.

（2） Tagged: The tagged port can be added in multiple VLANs. If a VLAN-tagged packet arrives at a port and the VLAN ID in its VLAN tag does not match any of the VLAN the

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