VLANs

SunilKhanna · ‎07-02-2009

Difference between LAN and VLAN?
When do you need a VLAN?
Why not to use subnetting for network?
How can devices on different VLAN’s communicate?
Configuration Example:
Related Information:

Difference between LAN and VLAN?

A LAN is a local area network and is defined as all devices in the same broadcast domain. If you remember, routers doesn't forward broadcast packets, switches just forward them.

VLAN stands for Virtual Local Area Network. VLANs (Virtual LAN) are a logical segmentation of LANs.In one physical cisco switch; you can create multiple VLANs that connect to different network.

When do you need a VLAN?

You need to consider using VLAN’s in any of the following situations:

    1) You have many devices on your LAN and want to decrease broadcast traffic.
    2) Users in groups need more security.
    3) Groups of users need to be on the same broadcast domain because they are running the same applications.

Why not to use subnetting for network?

Each VLAN should be in its own subnet. The benefit that a VLAN provides over a subnetted network is that devices in different physical locations, not going back to the same router, can be on the same network. The limitation of subnetting a network with a router is that all devices on that subnet must be connected to the same switch and that switch must be connected to a port on the router.

How can devices on different VLAN’s communicate?

Devices on different VLAN’s can communicate with a router or a Layer 3 switch. As each VLAN is its own subnet, a router or Layer 3 switch must be used to route between the subnets.

Configuration Example:

VLANs are created by number. The two ranges of VLANs are as follows:

1) The standard range consists of VLANs 1 to 1000.

2) The extended range consists of VLANs 1025 to 4096.

Every switch comes with the default VLAN 1 and all L2 the ports of switch are part of the default VLAN 1.The VLAN configuration steps for all the Catalyst switches are practically identical

Here I am taking an example of creating VLANs by functions (Sales, Operations and Finance) and more importantly I am expecting users in a particular function able to communicate with other members of the same function/VLAN but not with members of other function.

My first step is to create the three VLANS and then assign ports to respective vlan.

SW#config t

SW(config)#vlan 10

SW(config-vlan)#name Sales

SW#config-vlan)#vlan 11

SW(config-vlan)#name Operations

SW(config-vlan)#vlan 12

SW(config-vlan)#name Finance

SW(config-vlan)#exit

SW#sh vlan

VLAN Name Status Ports

1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4, Fa0/5, Fa0/6, Fa0/7, Fa0/8, Fa0/9, Fa0/10,

Fa0/11, Fa0/12, Fa0/13, Fa0/14, Fa0/15, Fa0/16,Gi0/1

10 Sales active

11 Operations active

12 Finance active

VLANS created namely Sales, Operations and Finance

Now we need to assign ports to respective VLANS

SW#config t

SW(config)#int fa0/1