Vlan Overview; Introduction To Vlan; Chapter 1 Vlan Overview - Huawei Quidway S3900 Series Operation Manual

Operation Manual -- VLAN
Quidway S3900 Series Ethernet Switches-Release 1510

1.1 VLAN Overview

1.1.1 Introduction to VLAN

The traditional Ethernet is a flat network, where all hosts are in the same broadcast
domain and connected with each other through hubs or switches. The hub is a physical
layer device without the switching function, so it forwards the received packet to all
ports. The switch is a link layer device which can forward the packet according to the
MAC address of the packet. However, 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. In this case, a host in the network receives a lot of packets whose
destination is not the host itself. Thus, plenty of bandwidth resources are wasted,
causing potential serious security problems.
The traditional way to isolate broadcast domains is to use routers. However, routers are
expensive and provide few ports, so they cannot subnet the network particularly.
The virtual local area network (VLAN) technology is developed for switches to control
broadcast in LANs.
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 with each other as if they are in a LAN. However, hosts in different VLANs
cannot communicate with each other directly. Figure 1-1 illustrates a VLAN
implementation.
LAN Switch LAN Switch
Figure 1-1 A VLAN implementation

Chapter 1 VLAN Overview

VLAN A VLAN A
VLAN B VLAN B
LAN Switch LAN Switch
VLAN A VLAN A
VLAN B VLAN B
Router Router
Huawei Technologies Proprietary
VLAN A VLAN A
VLAN B VLAN B
1-1
Chapter 1 VLAN Overview