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VLANs Virtual Local Area Networks Lesson No:33


 To understand the VLAN

Switch is more intelligent in the unicast communication because due to its Operating System it can learn the MAC table.

Let us suppose that there is a Switch Based network. Some computers a connected to switch

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Let the switch is connected to two departments one is Production department and second is the Sales department

The network ID of Production department is and network ID of Sales department is

If a computer P1 of Production department want to send broadcast to all other computers P2 P3 and P4 when this broadcast comes to the switch it will send this broadcast out from its all ports and this massage will received by all the computers connected to the switch (Production department Computers and Sales department Computers).

Computer P1 will write the broadcast address in the header of the massage and write the broadcast MAC address FFFF in the header.

When this broadcast comes to the all the computers from passing through the switch they all process this broadcast on the Physical Layer and we know that Physical Layer is not intelligent it will send this broadcast to the Data Link Layer.

At Data Link Layer all the computer will accept this broadcast after reading the broadcast MAC address FFFF and will thought that this massage is for us.

But when this massage further pass to the Network Layer of all the computers all the computers will read the broadcast IP address which is and only the computers of network (Production Department) realize that this broadcast is only for us. And all the computers of network will discard this broadcast on the Network Layer and this broadcast will only accepted by the computer of Production department, and they will further process this broadcast.


If a computer broadcast a frame it will write the broadcast IP address in its header what will be the broadcast MAC address?


The broadcast MAC address always will FFFF. It is MAC address of 12 hexadecimal digits. It will be understood by all Ethernet devices.


Hence the massage is discarded by the computer of Sale Department on the Network Layer.

This problem is only in the case of the Broadcast Massaging, because the Broadcast MAC address is always FFFF and it is understood by the switch that this massage must be send out from its all ports.


In the case of the unicast massaging the Switches has a MAC table of all the computers connected to its all ports. When one computer sends a massage to a specific computer it will write its specific MAC address and IP address (not the Broadcast MAC or Broadcast IP Address).

Switch read the MAC address and realize that this MAC address is out of my this Port and it will sent this massage only out from this port, and the computer connected to this port accept the massage.

So we can say that the switch is sending out the Unicast massage from its specific port. Not from all ports.


VLANs Virtual Local Area Networks)


If we do something such as

When a computer of the Network sends a Broadcast massage for only the computers of its own network, and when this massage come to the switch it do not send it out from its all ports except those relevant ports to which the computers of same network are connected. This Technique is called the formation of VLANs (Virtual Local Area Networks).

A physical LAN is divided into further Virtual LANs (logical LAN) so that each device in the LAN imagines that it is in different LAN.

For configuration of VLANs on the switch we make sure that those computers which are connected to the switch have same Network ID and we configure a specific VLAN ID against this network.


In the above topology there are eight computers which are connected to the switch. Four computers have Network ID and four have Network ID

If a computer generates a broadcast massage only for its own network this broadcast was received both Networks and and both process this broadcast to their Network Layer and relevant Network further access but not relevant network discard this broadcast.

But when we configure two VLAN ID against both networks then this broadcast will send out by the switch to the specific network only.

In other words we are restricting the switch to forward the specific broadcast out from that port to which the specific network is connected only.

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We have configured the VLAN ID on each port of switch for the specific network.

For network we have configured the VLAN ID è V10 and for the network we have configured the VLAN ID è V20 on all the relevant ports.

Now when a computer of any network generates a broadcast for its own network when this massage comes to the switch, switch see that from which port I am receiving this broadcast is member of which VLAN and switch will send out this broadcast only those ports to which this VLAN ID is matched. Now the traffic is controlled.


Subnets on a Network Layer are Ebullient to VLAN on a Data Link Layer. All the Ports of switch have by default VLAN ID -> V1

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If there are two switches (Switch1 and Switch2) are connected with each other and at switch one we have configure VLAN ID=V10 for Network and VLAN ID=V20 for Network Switch 2 is connected to its any port can the broadcast of Network and Network enter into the switch 2.

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The traffic generated from both networks cannot enter into the switch 2 because the VLAN IDs configured on the ports are V10 and V20 and the port of switch 2 have no any VLAN so its VLAN ID is by default = V1 so the traffic cannot enter to the Switch 2.

If we want that the traffic of both networks enter into the switch 2 we should establish two links and make member one Link of V10 and second Link of V20. Configure the VLAN ID on both ports V10 and V20 now the traffic of VLAN 10 and VLAN 20 can enter into the switch 2.

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There are two types of Links

1                   Access Link   Link between Switch and Computer

2                   Trunk Link     Link between Switch and Switch

Trunk Link


When number of networks increases on switch 1 then we have to increase the number of Links between Switch 1 and Switch 2 for each VLAN then this situation is becomes complicated.

So to avoid from this situation we use a specific type of Link which can move the traffic from all the networks of on switch to the other switch. This type of Link is called Trunk Links.


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When switch one send the traffic of V10, V20, V50 and V60 to the Switch 2 through the Single Trunk Link then there should any Protocol which separate the traffic of different VLANs in the Switch 2 and send traffic to the relevant VLAN.

Protocols are used for it these are called Trunk Encapsulation.

There are two types of Protocols:

No Protocols Header Size Developed By Supported By
1 802.1Q 4-Bytes IEEE Standardized All Vendors
2 ISL (Inter-Switch Link 30-Bytes Cisco Only Cisco Switches



If two switches are connected with each other and both are of Cisco then the encapsulation by default is ISL (Inter-Link Switch).

If one switch is of other company and second is of Cisco then 802.1Q encapsulation is used.

When we connect two switches then the Link in between is auto Trunk Link.

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