Chapter 15 Vlan; Vlan Overview; Tagged Vlans (Ieee 802.1Q) - ZyXEL Communications IES-1000 User Manual

15.1

VLAN Overview

A VLAN (Virtual Local Area Network) allows a physical network to be partitioned into multiple logical networks.
Stations on a logical network belong to one group. A station can belong to more than one group. With VLAN, a
station cannot directly talk to or hear from stations that are not in the same group(s); the traffic must first go
through a router.
In MTU applications, VLAN is vital in providing isolation and security among the subscribers. When properly
configured, VLAN prevents one subscriber from accessing the network resources of another on the same LAN, thus
a user will not see the printers and hard disks of another user in the same building.
VLANs also increase network performance by limiting broadcasts to a smaller and more manageable logical
broadcast domain. In traditional switched environments, all broadcast packets go to each and every individual port.
With VLAN, all broadcasts are confined to a specific broadcast domain.
Note that VLANs are unidirectional- they only govern outgoing traffic.
15.2

Tagged VLANs (IEEE 802.1Q)

When a LAN bridge receives a frame from a workstation, the VLAN from whence it came must be known so the
bridge may respond, if necessary, to the source of the frame. This is accomplished by tagging. There are two kinds
of tagging:
1. Explicit Tagging
•
A VLAN identifier is added to the frame header that identifies the source VLAN.
2. Implicit Tagging
•
The MAC (Media Access Control) number, the port or other information is used to identify the source of a
VLAN frame.
The IEEE 802.1Q Tagged VLAN uses both explicit and implicit tagging.
Tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of a frame
across bridges - tagged VLANs are not confined to the switch on which they were created. The VLANs can be
created statically by hand or dynamically through GVRP. The VLAN ID associates a frame with a specific VLAN
and provides the information that switches need to process the frame across the network. A tagged frame is four
bytes longer than an untagged frame and contains two bytes of TPID (Tag Protocol Identifier, residing within the
type/length field of the Ethernet frame) and two bytes of TCI (Tag Control Information, a tagged header starts after
the source address field of the Ethernet frame).
VLAN
This chapter explains how to configure VLANs on the IES-1000.
IES-1000 User's Guide
Chapter 15
VLAN
15-1