VLAN (Virtual Local Area Network)
Ports of a ethernet switch are group together to form Virtual Local Area Network referred as VLAN. In simple terms, VLAN is a group of switch ports that will function as though they are an independent ethernet switch. This is achieved by manipulating the frame forwarding software in the ethernet switch and adding a field in typical ethernet frame.
Manufacturers providing support of VLAN on ethernet switch will provide management software interface for network engineer or user to configure ports to be used in a VLAN.
Figure-1 depicts VLAN network. As shown an eight port ethernet switch can be configured as two VLANs as designated with VLAN-100 and VLAN-200. Ports 1 to 4 are configured in VLAN 100 and ports 5 to 8 in VLAN-200. Ethernet packets can be sent from station A to station B, but not from station A to stations C and D. As these VLANs act as separate networks, a broadcast or multicast sent on VLAN-100 will not be transmitted on any ports belonging to VLAN-200. The VLANs behave as though you had split the eight-port switch into two independent fourport switches.
VLAN Standard
The 802.1Q standard defines a virtual LAN (VLAN) as one or more switch ports that function as a separate and independent Ethernet system on a switch. Ethernet traffic within a given VLAN for example VLAN-100 will be sent and received only on those ports of the switch that are defined to be members of that particular VLAN. A 4 byte long Q-tag is inserted in an Ethernet frame between the source address and the type/length field to identify the VLAN to which the frame belongs. When a Q-tag is present, the minimum data field size is reduced to 42 bytes, maintaining a minimum ethernet frame size of 64 bytes. figure depicts VLAN Ethernet Frame mentioning the same with difference from typical ethernet frame.
VLAN ethernet frame
Switches can be connected together with an Ethernet segment that functions as a trunk connection that carries Ethernet frames with VLAN tags in them. This makes it possible for Ethernet frames belonging to VLAN 100, for example, to be carried between multiple switches and sent or received on switch ports that are assigned to VLAN 100.
The development of the IEEE 802.1Q standard for virtual bridged LANs produced the VLAN tag as a vendor-neutral mechanism for identifying which VLAN a frame belongs to. The addition of the 4 byte VLAN tag causes the maximum size of an Ethernet frame to be extended from the original maximum of 1518 bytes(not including the preamble) to a new maximum of 1522 bytes. Because VLAN tags are only added to Ethernet frames by switches and other devices that have been programmed to send and receive VLAN tagged frames, this does not affect traditional, or "classic", Ethernet operation. The first two bytes of the Q-tag contain an Ethernet type identifier of 0x8100. If an Ethernet station that is not programmed to send or receive a VLAN tagged frame happens to receive a tagged frame, it will see what looks like a type identifier for an unknown protocol type and simply discard the frame. VLANs and the contents and organization of VLAN tags are described
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