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GS-3012/GS-3012F User’s Guide
VLAN 7-1
Chapter 7
VLAN
The type of screen you see here depends on the VLAN Type you selected in the Switch Setup screen.
This chapter shows you how to configure 802.1Q tagged and port-based VLANs. See the General,
Switch and IP Setup chapter for more information.
7.1 Introduction to IEEE 802.1Q Tagged VLAN
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 (212) 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 number of VLAN configurations is 4,094.
TPID
2 Bytes User Priority
3 Bits CFI
1 Bit VLAN ID
12 bits
7.1.1 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 802.1Q
VLAN-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.
7.1.2 Automatic VLAN Registration
GARP and GVRP are the protocols used to automatically register VLAN membership across switches.