VLAN (Part 1)

What is a LAN?

Previously i said that a LAN is a group of devices (PCs, servers, routers, switches) in a single location (home office, etc.)

A more specific definition: A LAN is a single broadcast domain, including all devices in that broadcast domain.

Broadcast domains

A broadcast domain is the group of devices which will receive a broadcast frame (destination MAC FFFF.FFFF.FFFF) sent by any one of the members.

Perfomance: lots of unnecessary broadcast traffic can reduce network performance Security : Even within the same office you want to limit who has access to what. You can apply security policies on a router/firewall

Because this is one LAN, PCs can reach each other directly, without traffic passing through the router So even if you configure security policies, they won't have any effect.

Although we separated the three departments into three subnets (OSI), they are still in the same broadcast domain (OSI)

What is a VLAN

A switch will not forward traffic between VLANs, including broadcast/unknown unicast traffic

VLANs are configured on switches on a per-interface basis.
logically separate end hosts at Layer 2.

The switch does not perform inter-Vlan routing. It must send the traffic through the router

VLAN configuration

to display all the VLAN's available you can do the command

SW1#show vlan brief

By default all interfaces are set to VLAN 1

But VLANs1, 10002-1005 exist by default and cannot be deleted

to assign interfaces to a VLAN

SW1(config)#interface range g1/0 - 3
SW1(config)#switchport mode access
SW1(config)#switchport access vlan 10
% Access VLAN does not exist. Creating vlan 10

An access port is a switchport which belongs to asingle VLAN, and usually connects to end hosts like PCs

Switchports which carry multiple VLANs are called 'trunk ports' (More information on trunks in the next lesson)

You can change the name of the VLAN with the command :

SW1(config)#vlan 10
SW1(config-vlan)#name ENGINEERING

This Commang creates a VLAN. (In this case it was already automatically created when we assigned the interfaces)

Review

What is a (LAN)?
Broadcast domains
What is a Vlan
What is the purpose of VLANs?
How to configure VLANs on Cisco switches

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