VLAN! VLAN!! VLAN !!!

What is this VLAN ?

Why we are using  VLAN?

How to decide VLAN?

These are the question in my mind making storm… But when I took admission in RST forum. Vagish Sir cleared my all doubts about VLAN. So in this blog we will just focus on the “VLAN”

So 1^st question comes to your mind that why VLAN???

As consider a company like Accenture or Cisco in which there are 10000 of PC`s  and they are connected on a network and  all are in same subnet. The windows operating system is working on each computer. So as the windows operating system is always broadcasting its every service and any new service. So if we consider that our all PC`s are in a same subnet so the every PC will sends its service broadcast and when it is coming towards a switch it will broadcast it we can`t use router as all PC`s are in a same subnet. Switch will divide only the collision domain and not the broadcast domain so switch will make a multiple copies of that and send to its every port and so if 1 switch has 24 ports then the 10000/24 equal to a approximately 400 switches are there in a network. Lets imagine that for every broadcast the switch will make multiple copies and send its to every port and so it creates very high no of frames flowing through the network and so if now any PC want to use network then the network is already having very large traffic.so such a high no. of frames present in a network causes :-

• Degradation of switch performnce
• Degradation of PC`s performance
• Degradation of useful Bandwidth of a link

 so basically VLAN`s ARE USED TO CONTROL THE BROADCAST DOMAIN.

As so we divide the network in vlan, each vlan is having different subnet so for the communication between VLAN we use layer 3 switch or router.

We are doing VLAN configuration on a switch means what we are doing?

When we define a VLAN on a switch at that time we are actually dividing the switch in to the smaller swich virtually where these virtual  switchs are working in the different network (subnet).

From the following diagram you will get the concept …..

As we made VLAN`s so now each VLAN have its separate subnet so according to our basic principle the traffic between two subnet will route and traffic within a subnet will switch. A router or a layer 3 switch is necessary for communication between two VLAN`s.  

           How to decide that our which PC`s will be in one VLAN and which will be in the separate VLAN ???

These  is decided on the 80-20% rule .

This rule says that 80% traffic in a network should switch and 20% traffic should route.

As in a organization people which are in a same department talks more to each other and communicate more where as the people from different department will talk less to each other and communicate less to each other.

As we know switching is faster than routing. so the traffic within a department has to switch (as more traffic is there) and between the department should route (as less traffic is there).

So lets take a example that there is one company like cisco, in this company we have 4 different departments like engineering, Finance, Human resource and testing so as per our discussion we will keep the people from 1 department in a same VLAN so when they communicate then the traffic between them will switch but when communicate with other department (as they communicate with other department rarely ) so the traffic will route

Advantages of VLAN :-

• To controls the broadcast domain

• Use to provide security (it is actually a byproduct)

To provide security means not like the people from different can’t talk the can talk but their call or data will routed through a router so with the help of access list and some other way we can provide them the security. Some people think this is the main requirement of creating a VLAN but actually it is just a byproduct.(providing security to one department from other department doesn`t mean that they should not communicate to each other otherwise in above Cisco company example if a engineering dept. person can`t talk to testing dept. people then what is the use of the network)

All this is a basics of VLAN and after this we will see how the VLAN is configure on a cisco switch and can be extended on more than 1 switch in my next blog.If you have any doubt about VLAN you can ask it to me by comment. I will shortly publish my Blog on VLAN trunk port concept and VLAN Trunking Protocol (VTP). Thanks for reading. 

The following Diagram is added for the comment from himanshu to explain how broadcasting work in Switches on which VLAN is configured.