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6.3.2. 802.1Q VLAN
A virtual LAN, commonly known as a VLAN, is a group of hosts with a common set of
requirements that communicate as if they were attached to the Broadcast domain,
regardless of their physical location. A VLAN has the same attributes as a physical LAN,
but it allows for end stations to be grouped together even if they are not located on the same
network switch. Network reconfiguration can be done through software instead of
physically relocating devices.
VID - VLAN ID is the identification of the VLAN, which is basically used by the standard
802.1Q. It has 12 bits and allow the identification of 4096 (2^12) VLANs. Of the 4096
possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is
reserved, so the maximum possible VLAN configurations are 4,094.
A tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN
membership of a frame across bridges - they are not confined to the switch on which they
were created. The VLANs can be created statically by hand or dynamically through GVRP.
The VLAN ID associates a frame with a specific VLAN and provides the information that
switches need to process the frame across the network. A tagged frame is four bytes longer
than an untagged frame and contains two bytes of TPID (Tag Protocol Identifier, residing
within the type/length field of the Ethernet frame) and two bytes of TCI (Tag Control
Information, starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet
switches. If a frame received at an Ethernet port has a CFI set to 1, then that frame should
not be forwarded as it is to an untagged port. The remaining twelve bits define the VLAN
ID, giving a possible maximum number of 4,096 VLANs. Note that user priority and
VLAN ID are independent of each other. A frame with VID (VLAN Identifier) of null (0)
is called a priority frame, meaning that only the priority level is significant and the default
VID of the ingress port is given as the VID of the frame. Of the 4096 possible VIDs, a VID
of 0 is used to identify priority frames and value 4095 (FFF) is reserved, so the maximum
possible VLAN configurations are 4,094.
TPID User Priority CFI VLAN ID
2 bytes 3 bits 1 bit 12 bits

Forwarding Tagged and Untagged Frames
Each port on the Switch is capable of passing tagged or untagged frames. To forward a
frame from an 802.1Q VLAN-aware switch to an 802.1Q VLAN-unaware switch, the
Switch first decides where to forward the frame and then strips off the VLAN tag. To
forward a frame from an 802.1Q VLAN-unaware switch to an 802.1QVLAN-aware switch,
the Switch first decides where to forward the frame, and then inserts a VLAN tag reflecting
the ingress port's default VID. The default PVID is VLAN 1 for all ports, but this can be
changed.