Allied Telesis AT-iMG634 - R2 User's Guide download pdf (Page 235)

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Overview VLAN

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iMG/RG Software Reference Manual (Switching)

Many Ethernet switches support virtual LAN (VLAN) technologies. By replacing hubs with VLAN switches, the

network administrator can create a virtual network within existing network. With VLAN, the network logical

topology is independent of the physical topology of the wiring. Each computer can be assigned a VLAN identifi-

cation number (ID), and computers with the same VLAN ID can act and function as though they are all on the

same physical network.

So, the traffic on a VLAN is isolated and thus all communications remain within the VLAN. The assignment of

VLAN IDs is done by the switches and can be managed remotely using network management software.

VLAN switches can function in different ways. They can be switched at the data-link layer (layer 2 of the Open

Systems Interconnection reference model) or the network layer (layer 3), depending on the type of switching

technology used. The main advantage of using VLAN technologies is that users can be grouped together accord-

ing to their need for network communication, regardless of their actual physical locations. This isolation will

help to reduce unnecessary traffic so better network performance. The disadvantage is that additional configu-

ration is required to set up and establish the VLANs when implementing these switches.

2.4.1.1 VLAN tagging

VLAN technology introduces the following three basic types of frame:

• Untagged frames

• Priority-tagged frames

• VLAN-tagged frames

An untagged frame or a priority-tagged frame does not carry any identification of the VLAN to which it belongs.

Such frames are classified as belonging to a particular VLAN based on parameters associated with the receiving

port.

This classification mechanism requires the association of a specific VLAN ID, the Port VLAN Identifier, or PVID,

with each of the switch ports.

The PVID for a given port provides the VID for untagged and priority-tagged frames received through that port.

The PVID for each port shall contain a valid VID value, and shall not contain the value of the null VLAN ID (see

Table 8)

A VLAN-tagged frame carries an explicit identification of the VLAN to which it belongs; i.e., it carries a non-null

VID. Such a frame is classified as belonging to a particular VLAN based on the value of the VID that is included

in the tag header. The presence of a tag header carrying a non-null VID means that some other device, either

the originator of the frame or a VLAN-aware switch, has mapped this frame into a VLAN and has inserted the

appropriate VID.

Tagging of frames is performed for the following purposes:

• To allow user priority information to be added to frames carried on IEEE 802 LAN MAC types that have no

inherent ability to signal priority information at the MAC protocol level;

• To allow a frame to carry a VID;

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