Figure 3-1shows a typical VLAN application. Three switches are placed in different locations
(for example, in different floors of a building); each switch is connected to three hosts that
respectively belong to different VLANs (for example, different companies).
Benefits
The VLAN technology brings the following benefits to customers:
l Limits broadcast domains. A broadcast domain is limited in a VLAN. This saves bandwidth
and improves network processing capabilities.
l Enhances network security. Packets from different VLANs are separately transmitted.
Hosts in a VLAN cannot directly communicate with hosts in another VLAN.
l Improves network robustness. A fault in a VLAN does not affect hosts in other VLANs.
l Flexibly sets up virtual groups. With the VLAN technology, hosts in different geographical
areas can be grouped together. This facilitates network construction and maintenance.
3.2 Principles
This section describes principles of VLAN.
3.2.1 Basic Concepts of VLAN
VLAN frame format
A conventional Ethernet frame is encapsulated with the Length/Type field for an upper-layer
protocol following the Destination address and Source address fields, as shown in Figure 3-2.
Figure 3-2 Conventional Ethernet frame format
2bytes6bytes 6bytes 46-1500bytes 4bytes
Destination
address
Source
address
Length/Type
Data FCS
IEEE 802.1Q is an Ethernet networking standard for a specified Ethernet frame format. It adds
a 4-byte field between the Source address and the Length/Type fields of the original frame, as
shown in Figure 3-3.
Figure 3-3 802.1Q frame format
2bytes 3bits 12bits1bit
4bytes
2bytes
802.1Q
Tag
Data FCS
TPID PRI CFI VID
6bytes 6bytes 46-1500bytes 4bytes
Destination
address
Source
address
Length/
Type
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
3 VLAN Configuration
Issue 01 (2014-11-30) Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
72
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