Virtual LAN (VLAN) is a group of devices on one or more LANs that are configured so that they can communicate as if they were attached to the same wire, when in fact they are located on a number of different LAN segments. Because VLANs are based on logical instead of physical connections, it is very flexible for user/host management, bandwidth allocation and resource optimization.

There are the following types of Virtual LANs:

1. Port-Based VLAN: each physical switch port is configured with an access list specifying membership in a set of VLANs.

2. MAC-based VLAN: a switch is configured with an access list mapping individual MAC addresses to VLAN membership.

3. Protocol-based VLAN: a switch is configured with a list of mapping layer 3 protocol types to VLAN membership C thereby filtering IP traffic from nearby end-stations using a particular protocol such as IPX.

4. ATM VLAN C using LAN Emulation (LANE) protocol to map Ethernet packets into ATM cells and deliver them to their destination by converting an Ethernet MAC address into an ATM address.

The IEEE 802.1Q specification establishes a standard method for tagging Ethernet frames with VLAN membership information. The IEEE 802.1Q standard defines the operation of VLAN Bridges that permit the definition, operation and administration of Virtual LAN topologies within a Bridged LAN infrastructure. The 802.1Q standard is intended to address the problem of how to break large networks into smaller parts so broadcast and multicast traffic would not grab more bandwidth than necessary. The standard also helps provide a higher level of security between segments of internal networks.

The key for the IEEE 802.1Q to perform the above functions is in its tags. 802.1Q-compliant switch ports can be configured to transmit tagged or untagged frames. A tag field containing VLAN (and/or 802.1p priority) information can be inserted into an Ethernet frame. If a port has an 802.1Q-compliant device attached (such as another switch), these tagged frames can carry VLAN membership information between switches, thus letting a VLAN span multiple switches. However, it is important to ensure ports with non-802.1Q-compliant devices attached are configured to transmit untagged frames. Many NICs for PCs and printers are not 802.1Q-compliant. If they receive a tagged frame, they will not understand the VLAN tag and will drop the frame. Also, the maximum legal Ethernet frame size for tagged frames was increased in 802.1Q (and its companion, 802.3ac) from 1,518 to 1,522 bytes. This could cause network interface cards and older switches to drop tagged frames as "oversized."

Protocol Structure

IEEE 802.1Q Tagged Frame for Ethernet:

• Preamble (PRE) - 7 bytes. The PRE is an alternating pattern of ones and zeros that tells receiving stations that a frame is coming, and that provides a means to synchronize the frame-reception portions of receiving physical layers with the incoming bit stream.
• Start-of-frame delimiter (SFD) - 1 byte. The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits indicating that the next bit is the left-most bit in the left-most byte of the destination address.
• Destination address (DA) - 6 bytes. The DA field identifies which station(s) should receive the frame.
• Source addresses (SA) - 6 bytes. The SA field identifies the sending station.
• TPID - defined value of 8100 in hex. When a frame has the EtherType equal to 8100, this frame carries the tag IEEE 802.1Q / 802.1P.
• TCI C Tag Control Information field including user priority, Canonical format indicator and VLAN ID.

• User Priority : Defines user priority, giving eight (2^3) priority levels. IEEE 802.1P defines the operation for these 3 user priority bits.
• CFI : Canonical Format Indicator is always set to zero for Ethernet switches. CFI is used for compatibility reason between Ethernet type network and Token Ring type network. 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.
• 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.
• Length/Type- 2 bytes. This field indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format.
• Data-Is a sequence of n bytes (42=< n =<1496) of any value. The total frame minimum is 64bytes.
• Frame check sequence (FCS)- 4 bytes. This sequence contains a 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

Related protocols:IEEE 802.3, 802.2, 802.1D, 802.1p

Sponsor Source: VLAN standard is defined by IEEE (http://www.ieee.org) 802.1Q.

Reference: http://standards.ieee.org/getieee802/download/802.1Q-1998.pdf: IEEE 802.1Q Standard