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CCNA: VLAN - Virtual Local Area Networks

Some basic knowledge of LANs, different topologies, and working of Local Area Networks is required to proceed further.

What is a VLAN?

        To refresh your memory, a Local Area Network (LAN) is a set of connected devices like computers, hubs, and switches sharing the same pool of logical address space. Normally, a router is required to route packets from one LAN to another LAN. Traditionally, all packets within a LAN are broadcast to all other devices connected to that particular LAN. 

As a result, a traditional LAN has several disadvantages as below:

  • Usable bandwidth is shared among all the devices connected to the LAN
  • ALL devices connected within a LAN can hear ALL the packets irrespective of whether the packet is meant for that device or not. It is possible for some unscrupulous node listening to data packets not meant for that.
  • Suppose, your organization has different departments. Using a traditional LAN, when any changes take place within the organization, physical cables and devices need to be moved to reorganize the LAN infrastructure.
  • A LAN cannot extend beyond its physical boundary across a WAN as in VLANs.

If you are looking for a simple networked solution for a small office, it may be a good idea to have a traditional LAN setup with a few hubs or switches. However, if you are planning for a large building or campus wide LAN for several individual departments, a VLAN is almost essential.

Virtual LANs (VLANs) can be considered as an intelligent LAN consisting of different physical LAN segments enabling them to communicate with each other as if they were all on the same physical LAN segment.

Benefits of VLAN: Several of the disadvantages of traditional LANs can be eliminated with the implementation of VLANs.

1. Improved Performance: In a traditional LAN, all the hosts within the LAN receive broadcasts, and contend for available bandwidth. As a result, the bandwidth is shared among all the connected devices within the LAN segment. If you are running high-bandwidth consumption applications such as groupware or server forms, a threshold point may easily be reached. After a threshold, the users may find the LAN too slow or un-responsive. With the use of VLAN, you can divide the big LAN into several smaller VLANs. For example, if there are two file servers, each operating at 100Mbps, in a traditional LAN both the servers have to share the LAN bandwidth of 100Mbps. If you put both the servers in separate VLANs, then both have an available bandwidth of 100Mbps each. Here the available bandwidth has been doubled.

2. Functional separation of an institute or a company: It is often required to separate the functional groups within a company or institute. For example, it might be necessary to separate HR department LAN from that of Production LAN. Traditionally, it requires a router to separate two physical LANs. However, you can set up two VLANs, one for Finance, and the other for Production without a router. A switch can route frames from one VLAN to another VLAN. With VLAN's it is easier to place a workgroup together eventhough they are physically in different buildings. In this case Finance VLAN does not forward packets to Production VLAN, providing additional security. 

3. Ease of Network Maintenance: 

Network maintenance include addition, removal, and changing the network users. With traditional LANs, when ever a User moves, it may be necessary to re-configure the user work station, router, and the servers. Some times, it may also be necessary to lay the cable, or reconfigure hubs and switches.  If you are using VLANs, many of these reconfiguration tasks become unnecessary. For example, you can avoid network address configuration on the work station and the corresponding router if you use VLAN. This is because, routing traffic within VLANs doesn't require a router.  

However, VLAN's add some administrative complexity, since the administration needs to manage virtual workgroups using VLANs. 

4. Reduced Cost

VLANs minimize the network administration by way of reduced maintenance on account of workstation addition/deletion/changes. This in turn reduce the costs associated with LAN maintenance.

5. Security

Using a LAN, all work stations within the LAN get the frames meant for all other work stations within the broadcast domain. Since a VLAN splits the broadcast domain into two or more, it is possible to put work stations sharing sensitive data in one VLAN, and other  work station in another VLAN. Of course, if two VLANs are not sufficient, you can split the work stations into as many VLANs as required. VLAN's can also be used to set up firewalls, restrict access, and send any intrusion alerts to the administrator. 


Question: Your network has 100 nodes on a single broadcast domain. You have implemented VLANs and segmented the network to have 2 VLANs of 50 nodes each. The resulting broadcast traffic effectively:

A. Increases two fold

B. Remains same

C. Decreases by half

D. Increases 4 fold

Ans: C

Explanation: By implementing VLANs, the effective broadcast traffic decreases, since VLANs do not forward the broadcast traffic from one VLAN to another.


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