VLAN

Virtual LAN, Surprise!

     This is the last lecture topic for 3^rd year students who learn Data and Computer Communication in first semester course of Computer University. During first semester, we all were busy. So we couldn’t post any blog articles to the page. Now I would like to post an article for our students.

     Before describe VLAN, we need to know Ipv4 addressing. IPv4 allow three types of addresses:

                               i.            Unicast

                             ii.            Multicast

                          iii.            Broadcast

  Unicast:  An identifier for a single interface. If you have a unique destination address, your MAC frame can be forwarded to your intended destination station.

Multicast:   An identifier for a set of interfaces. If you want to send a frame for a set of interfaces or a group, you can insert group address in destination address field of frame header. This frame is delivered to all interfaces identified by that group address.

Broadcast:  Sometimes a computer wants to send the same frame to each and 

every computer attached to the common transmission medium. Sending a frame to all the other computers is called broadcasting. So, A MAC frame’s destination address may also contain broadcast address (means all 1s in destination address field),in which case the destination MAC address indicates that all devices should receive a copy of the frame. When network devices (switch etc.) don’t know the incoming frame’s destination address, they can forward this MAC frame using broadcast address.

     Now, the reader will more understand about the addresses in IPv4. Sometime, broadcast address may waste transmission capacity on the other portions of the LAN and on the other switches. I mean the LAN with more than one switch.  As the number of workstations on the typical LAN grew, they started to become hopeless congested there were just too many collisions because most of the time when a workstation tried to send a packet, it would find that the wire was already occupied by a packet by some other device.

     One simple approach to improving efficiency is to physically partition the LAN into separate broadcast domains (the total collection of devices that receive broadcast frames from each other is referred to as a broadcast domain) using routers. In this method, as LANs became larger, data rates became faster, and users desired greater flexibility, the routers in a network started to become a bottleneck. This is because:

•  Routers typically forward data in software, and so are not as fast as switches

•  Splitting up a LAN using routers meant that a LAN typically corresponded to a particular physical location. This became limiting when many users had laptops, and wanted to be able to move between buildings, but still have the same network environment wherever they plugged in.

     In this article, I discuss a more effective alternative to reduce the transmission capacity wasted is the creation of Virtual LANs(VLANs). A VLAN is a logical subgroup within a LAN that is created by software rather than by physically moving and separating devices. It combines user stations and network devices (switches etc.) into a single broadcast domain without regard the physical location of users. You can assign each switch port to only one VLAN. Ports in a VLAN share broadcasts; ports in different VLANs do not share broadcasts. This improves the overall performance of the network. A VLAN can exist on a single switch or span multiple switches. It can include stations in a single building or multiple-building infrastructures, or it can connect across WANs. 

     Using VLAN technology, you can group switch ports and their connected users into logically defined communities, such as coworkers in the same department, a cross-functional product team, or diverse user groups sharing the same network application.

 

     The above figure shows three VLANs Sales, HR, ENG departments. A group of users to share a common broadcast for the same VLAN are depicted in figure. Some means is therefore needed for defining VLAN membership. A number of different approaches have been used for defining membership, including the following:

•  Membership by port group: Each switch in the LAN contains two types of port: a trunk port, which connects two switches, and an end port, which connects the switch to an end system.A VLAN can be defined by assigning each end port to a specific VLAN. Normally, a port carries traffic only for the single VLAN to which it belongs. For a VLAN to span across multiple switches, a trunk is required to connect two switches. A trunk can carry traffic for multiple VLANs. Trunking is a way to carry traffic from several VLANs over a point-to-point link between the two devices.

•  Membership by MAC address: Since MAC-layer addresses are hardwired into the workstation’s network interface card (NIC), VLANs based on MAC addresses enable network managers to move a workstation to a different physical location on the network and have that workstation automatically retain its VLAN membership;

• Membership based on protocol information: VLAN membership can be assigned based on IP address, transport protocol information, or even higher-layer protocol information.
  

References:

1.    Data and Computer Communications , Ninth Edition by William Stallings

2.  Computer Networking (A Top-Down Approach) Fourth Edition by 
    James F. Kurose, Keith W.Ross

3.    AlliedWare Plus™ OS Overview: VLANs

4.    Interconnecting Cisco Network Devices (ICND) v2.1

More detail about VLANs may learn in open source (Internet)!