VLAN

Vlans (Virtual Local area Network) are used to divide a physical network into several broadcast domains.

The reason to use VLANs is to divide a network and separate hosts that shouldn't be able to access each other.

 

VLAN=SUBNET=Broadcast domain

 There are two types of packets on a VLAN:

•  Tagged
•  Untagged packets.

 The untagged packet is a regular packet and looks just like a packet that exists on a regular network. Untagged packets are the most common type on a VLAN. The decision of which VLAN an untagged packet belongs to is made by the switch. A switch can be configured to assign specific ports to specific VLANs. The switch can also be configured to receive tagged packets.

If the switch receives a tagged packet and the port which it receives the packet with is configured to allow tagged packets, it knows which ports it can send the packet to.

A switch can also be configured to transmit tagged packets, this could be used to make a VLAN span more than one switch or to make use of a VLAN aware NIC (Network Interface Card) on a router, firewall, server or even a workstation. 

A VLAN is assigned a specific id. This id can be anything between 1 and 4094.VLANs you created in the range of 1006-4094 are called Extended VLAN.
 VLAN 1 is most commonly used for management so this should not be used.


Why we want separation of VLAN:

Some time you don't have just one switch and you need to carry out outside you switch because you have plugs some other devices in an another area and this another switch need to know what is a VLANs he can forward and what is the port he can use.

The configuration of the switch can be the same of the configuration you have made in the other switch depend and what the port is used.

The name of the functionality to identified the Vlans in the particular port is mode access port for device and you bind (or allow)  the particular vlan in this ports.

But between the switch you need to have a functionality to forward all or particular VLANs between a different switch the name and technologies is a Trunking.

Trunking is used to carry the VLANs between the switch and if you capture the trame in this area you can see the tag of 802.1Q.


Let's configure 2 VLANs, 100 and 200, for accounting and marketing

We can see that they are created  using show vlan command


Now we want to populate VLANs with ports: We will assign fa0/3 to accounting department

We want to assign a group of ports together: fa0/5, 6 , 7, 8 assign to marketing

We verified that they were created with show vlan brief commnad

If we want to delete a VLAN

F0/3 is not really deleted but unusable at the time.We need to reassign it to VLAN 1

VLANs live in the flash memory, in its vlan database :  VLAN.DAT

If you really want to delete a database , to wipe out the switch configuration use:

#write erase
#delete flash:vlan.dat