VLAN Basics

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 end stations 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 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 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 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 to 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 802.1Qcompliant 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.

Advantage of VLAN

VLAN provides following advantages:-

• Solve broadcast problem
• Reduce the size of broadcast domains
• Allow us to add additional layer of security
• Make device management easier
• Allow us to implement the logical grouping of devices by function instead of location

Solve broadcast problem

When we connect devices to the switch ports, the switch creates separate collision domain for each port and single broadcast domain for all ports. Switch forwards a broadcast frame from all possible ports. In a large network having hundreds of computers, it could create a performance issue. Of course, we could use routers to solve a broadcast problem, but that would be a costly solution since each broadcast domain requires its own port on a router. A switch has a unique solution to broadcast issue known as VLAN. In the practical environment, we use VLAN to solve the broadcast issue instead of a router.

Each VLAN has a separate broadcast domain. Logically VLANs are also subnets. Each VLAN requires a unique network number known as VLAN ID. Devices with same VLAN ID are the members of the same broadcast domain and receive all broadcasts. These broadcasts are filtered from all ports on a switch that aren’t members of the same VLAN.

Reduce the size of broadcast domains

VLAN increase the numbers of the broadcast domain while reducing their size. For example, we have a network of 100 devices. Without any VLAN implementation, we have a single broadcast domain that contains 100 devices. We create 2 VLANs and assign 50 devices in each VLAN. Now we have two broadcast domains with fifty devices in each. Thus more VLAN means more broadcast domain with fewer devices.

Allow us to add additional layer of security

VLANs enhance the network security. In a typical layer 2 networks, all users can see all devices by default. Any user can see network broadcast and responds to it. Users can access any network resources located on that specific network. Users could join a workgroup by just attaching their system to existing switch. This could create real trouble on a security platform. Properly configured VLANs gives us total control over each port and users. With VLANs, you can control the users from gaining unwanted access to the resources. We can put the group of users that need high-level security into their own VLAN so that users outside from VLAN can’t communicate with them.

Make device management easier

Device management is easier with VLANs. Since VLANs are a logical approach, a device can be located anywhere in the switched network and still belong to the same broadcast domain. We can move a user from one switch to another switch in the same network while keeping his original VLAN.

Allow us to implement the logical grouping of devices by function instead of location

VLANs allow us to group the users by their function instead of their geographic locations. Switches maintain the integrity of your VLANs. Users will see only what they are supposed to see regardless what their physical locations are.

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