CCNA VLANS
As a CCNA student you must be familiar with and comfortable with the concepts of Vlans Before you get into what they are is you need to understand what a Local area network is.
The definition of a Local Area Network (LAN) is a collection of network devices located on a shared broadcast domain. This broadcast domain may comprise one physical backbone like a Co-axial cable with drop cables running back to the hosts on the LAN.
The number of devices on the shared broadcast domain will have an impact on the performance of your network. Hosts on the network are continually sending out broadcasts on discover other hosts on the local network segment. Broadcasts are a necessary evil on your networks, without them your hosts would and could not discover the layer 2 addresses of other machines on the local LAN.
This is where you start to consider implementing vlans on your network. With vlans operating on your networks you can control the scope or range of the broadcast and contain it and prevent it from affecting all host across your lans.
Not only can you prevent broadcasts from unnecessarily interrupting hosts you also use vlans to group hosts with similar functions into a common vlan for the purposes of security. Once a Host is on a vlan it is protected from seeing or being seen by devices on other vlans, even of the host on other vlan is on the next port along. Using vlans allows us to logical partition your switches.
Using vlans to logically partition your switches you do away with the need to purchase any further equipment to segment your network.
Lets look at partitioning the switch into logical segments. When the switch first arrives out of the box or is defaulted back to factory defaults all of the ports are in one common vlan known as VLAN 1. This entity is also referred to as the NATIVE VLAN.
When you connect your network hosts into the ports all subsequent traffic which they generate will be placed into the vlan of that connected port, in the default case this would be V1, all devices which are connected to this vlan are going to see all other hosts broadcast traffic, so here we see that by merely placing the devices into a vlan does not mean the broadcasts will cease.
We are going to use and example 24 port switch to see how to configure the device.
In this exercise the task is to create 3 additional vlans and place 8 ports into each vlan, the result ought to be that the switch has 3 new broadcast domain.
Switch#
Switch#configure terminal
Switch(config)#vlan 2
Switch(config-vlan)#exit
Switch(config)#vlan 3
Switch(config-vlan)#exit
Switch(config)#vlan 4
Switch(config-vlan)#exit
In the example above the commands used created and additional 3 vlans on our switch
Switch(config)#interface range fastethernet 0/1 - 8
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 2
Switch(config-if-range)#exit
Switch(config)#interface range fastethernet 0/9 - 16
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 3
Switch(config-if-range)#exit
Switch(config)#interface range fastethernet 0/17 - 24
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 4
In the commands above:
The ports numbered from 1 through 8 were assigned to v2
The ports numbered from 9 through 16 were assigned to v3
The ports numbered from 17 through 24 were assigned to v4
Switch(config-if-range)#end
Switch#copy run start
Finally we save our configuration.
In all our fictional switch now has 3 new broadcast domains, by populating all of our switchports with hosts it would have the effect of securing the visibility of host on one broadcast domain from seeing hosts on one of the other broadcast domains from a security stand point this is ideal and from a performance point of view it achieves exactly what we need
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