Netis ST3216 [20/42] Advantages of vlans

Introduction to VLAN

The traditional Ethernet is a broadcast network, where all hosts are in the same broadcast

domain and connected with each other through hubs or switches. Hubs and switches, which are

the basic network connection devices, have limited forwarding functions.

 A hub is a physical layer device without the switching function, so it forwards the received

packet to all ports except the inbound port of the packet.

 A switch is a link layer device which can forward a packet according to the MAC address of

the packet. A switch builds a table of MAC addresses mapped to associated ports with that

address and only sends a known MAC’s traffic to one port. When the switch receives a

broadcast packet or an unknown unicast packet whose MAC address is not included in the

MAC address table of the switch, it will forward the packet to all the ports except the

inbound port of the packet. The above scenarios could result in the following network

problems.

 Large quantity of broadcast packets or unknown unicast packets may exist in a network,

wasting network resources.

 A host in the network receives a lot of packets whose destination is not the host itself,

causing potential serious security problems.

 Related to the point above, someone on a network can monitor broadcast packets and

unicast packets and learn of other activities on the network. Then they can attempt to

access other resources on the network, whether or not they are authorized to do this.

Isolating broadcast domains is the solution for the above problems. The traditional way is to use

routers, which forward packets according to the destination IP address and does not forward

broadcast packets in the link layer. However, routers are expensive and provide few ports, so

they cannot split the network efficiently. Therefore, using routers to isolate broadcast domains

has many limitations.

The Virtual Local Area Network (VLAN) technology is developed for switches to control

broadcasts in LANs.

A VLAN can span multiple physical spaces. This enables hosts in a VLAN to be located in different

physical locations. By creating VLANs in a physical LAN, you can divide the LAN into multiple

logical LANs, each of which has a broadcast domain of its own. Hosts in the same VLAN

communicate in the traditional Ethernet way. However, hosts in different VLANs cannot

communicate with each other directly but need the help of network layer devices, such as

routers and Layer 3 switches.

Advantages of VLANs

Compared with traditional Ethernet technology, VLAN technology delivers the following benefits:

 Confining broadcast traffic within individual VLANs. This saves bandwidth and improves

network performance.