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802.1Q VLANs for better bandwidth

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The IEEE's 802.1Q standard was developed to address the problem of how to break large networks into smaller parts so broadcast and multicast traffic wouldn't grab more bandwidth than necessary. The standard also helps provide a higher level of security between segments of internal networks.

The 802.1Q specification establishes a standard method for inserting virtual LAN (VLAN) membership information into Ethernet frames.

In a LAN, datalink-layer broadcast and multicast traffic is delivered to all endstations, but this traffic cannot go beyond the LAN boundary. In the past, shared cabling or hubs were the boundaries for LANs.


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Because network protocols typically rely on broadcast queries to let endstations discover one another, devices on two LANs cannot "see" each other without the help of a network-layer device with ports in both LANs, such as a router.

The fact that broadcasts are distributed to all devices in a LAN means LANs cannot become very large. If they do, devices become overburdened with broadcast traffic. The ability of devices in a LAN to discover each other also means servers housing sensitive data should be placed in a LAN separate from the average user, with router filters controlling access. These factors make it critical for network administrators to control LAN boundaries.

A VLAN is an administratively configured LAN or broadcast domain. Instead of going to the wiring closet to move a cable to a different LAN, network administrators can accomplish this task remotely by configuring a port on an 802.1Q-compliant switch to belong to a different VLAN. The ability to move endstations to different broadcast domains by setting membership profiles for each port on centrally managed switches is one of the main advantages of 802.1Q VLANs.

The switch acts as an intelligent traffic forwarder and a simple network security device. Frames get sent only to the ports where the destination device is attached. Broadcast and multicast frames are constrained by VLAN boundaries so only stations whose ports are members of the same VLAN see those frames. This way, bandwidth is optimized and network security is enhanced.

802.1Q VLANs aren't limited to one switch. VLANs can span many switches, even across WAN links. Sharing VLANs between switches is achieved by inserting a tag with a VLAN identifier (VID) between one and 4,094 into each frame. A VID must be assigned for each VLAN. By assigning the same VID to VLANs on many switches, one or more VLAN (broadcast domain) can be extended across a large network.

The secret to performing this magic is in the 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.

There is one important caveat: Network administrators must ensure ports with non-802.1Q-compliant devices attached are configured to transmit untagged frames. Many network interface cards 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."

In the case of a network with an ATM WAN, Ethernet switches with ATM uplinks can have a VLAN-to-emulated-LAN (ELAN) mapping feature that matches 802.1Q VIDs to ATM ELAN names. This lets the benefits of VLAN bandwidth optimization and security be extended between campus buildings or even between remote sites.

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Green is a product line engineer in the Enterprise Business Systems Group of Marconi Communications. He can be reached at Dave.Green@marconi.com.

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