How VLANs work

A VLAN is a logical network segment that can span multiple physical network segments. The end-stations belonging to a VLAN are related by function or application.

For example, end-stations in a VLAN might be grouped by departments, such as engineering and accounting, or by projects, such as release1 and release2. Because physical proximity of the end-stations is not essential in a VLAN, you can disperse the end-stations geographically and still contain the broadcast domain in a switched network.

• VLAN membership
  An end-station must become a member of a VLAN before it can share the broadcast domain with other end-stations on that VLAN. The switch ports can be configured to belong to one or more VLANs (static registration), or end-stations can register their VLAN membership dynamically, with VLAN-aware switches.
• How VLAN membership affects communication
  Any broadcast or multicast packets originating from a member of a VLAN will be flooded only among the members of that VLAN. Communication between VLANs, however, must go through a router.
• GARP VLAN Registration Protocol
  The GARP VLAN Registration Protocol (GVRP) uses the Generic Attribute Registration Protocol (GARP) to allow end-stations on a network to dynamically register their VLAN membership with GVRP-aware switches. Similarly, these switches dynamically register with other GVRP-aware switches on the network, thus creating a VLAN topology across the network.
• VLAN tags
  A VLAN tag is a unique identifier that indicates the VLAN to which a frame belongs. Generally, a VLAN tag is included in the header of every frame sent by an end-station on a VLAN.
• Advantages of VLANs
  VLANs provide a number of advantages including ease of administration, confinement of broadcast domains, reduced network traffic, and enforcement of security policies.
• Prerequisites for setting up VLANs
  You need to satisfy certain prerequisites for switches and end stations before you can set up VLANs in a network: