Why we need VLANs, an Introduction to VLAN technology

The design of layer-2 switched network is somewhat like flat network. Each and every device on the newtork can see the transmission of every broadcast packet even if it does not need to receive the data.

vlan1 - flat network

The Structure of Flat Network

The routers permit the broadcasting within the originating network only but it does switch forward the broadcasts in each and every compartment or segment. It is called flat network not because of its flat design but due to the reason that it has a single broadcast domain. As shown in the figure the broadcasting by Host A is forwarded to all the ports on all switches leaving the port that received it in the beginning.

In the second figure you can see a switched network sending frame with Host A and Host D as its terminal/destination. You can notice that the frame has forwarded out only the port where the Host D is situated. This is a great advancement if you compare it with old hub networks but if you want one collision domain by default then you may not like it.  

vlan2 - flat network

So, the biggest advantage of layer-2 switched network is that it establishes a particular collision domain segment or compartment for every single equipment connected to the switch. As a result larger networks can be established and there is no compulsion of Ethernet distance anymore. That does not mean it is completely free from issues-when the number of devices and users is greater then each switch has to deal with more packets and broadcasts.

Security is also another issue within the typical layer-2 switched internetwork as all the devices are visible to all the users. The drawback is that it is not possible to stop the broadcast from devices as well as the response of the users to these broadcasts. Sadly, the choice of security is restricted when it comes to passwords placing on various servers as well as on other devices. But not if you establish a virtual LAN. On the other hand, many issues can be resolved that are  related with layer-2 switching with the help of VLANs. The VLANs makes network management easy with number of ways:

  • The VLAN can categorize many broadcast domains into number of logical subnets.
  • The network needs to configure a port into the suitable VLAN in order to achieve change, add or move.
  • In the VLAN a group of users with the demand of high security can be included so that the external users out the VLAN cannot interact with them.
  • When it comes to logical classfication of users in terms of function, we can consider VLAN as independent from their geographic or  physical locations.
  • Even the security of network can be enhanced by VLAN.
  • The number of broadcast domains are increased with VLANs while the size decreases.


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