Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 344)

Switching Introduction
Software Reference for x310 Series Switches
14.2 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
Introduction
This chapter gives an overview of Layer 1 and 2 switching.
Layer 2 switches are used to connect multiple Local Area Network (LAN) segments
together to form an extended LAN. Stations connected to different LANs can be
configured to communicate with one another as if they were on the same LAN. They can
also divide one physical LAN into multiple Virtual LANs (VLANs). Stations connected to
each other on the same extended LAN can be grouped in separate VLANs, so that a station
in one VLAN can communicate directly with other stations in the same VLAN, but must go
through higher layer routing protocols to communicate with those stations in other
VLANs.
Layer 2 switches appear transparent to higher layer protocols, transferring frames
between the data link layers of the networks to which they are attached. A Layer 2 switch
accesses each physical link according to the rules for that particular network. Access may
not always be instant, so the switch must be capable of storing and forwarding frames.
Storing and forwarding enables the switch to examine both the VLAN tag fields and
Ethernet MAC address fields in order to forward the frames to their appropriate
destination. In this way, the switch can act as an intelligent filtering device, redirecting or
blocking the movement of frames between networks.
Because switch ports can sometimes receive frames faster than it can forward them, the
switch has Quality of Service (QoS) queues in which frames await transmission according
to their priority. Such a situation could occur where data enters a number of input ports all
destined for the same output port.
The switch can be used to:
■ Increase both the physical extent and the maximum number of stations on a LAN.
LANs are limited in their physical extent by the signal distortion and propagation
delay characteristics of the media. The switch overcomes this limitation by receiving a
frame on one LAN and then retransmitting it to another. The physical characteristics
of the LAN media also place a practical limit on the number of stations that can be
connected to a single LAN segment. The switch overcomes this limitation by joining
LAN segments to form an extended LAN capable of supporting more stations than
either of the individual LAN segments.
■ Connect LANs that have a common data link layer protocol but different physical
media, for example, Ethernet 10BASET, 100BASET, and 10BASEF.
■ Increase the availability of LANs by allowing multiple redundant paths to be
physically configured and selected dynamically, using the Spanning Tree algorithm.
■ Reduce the load on a LAN or increase the effective bandwidth of a LAN, by filtering
traffic.
■ Prioritize the transmission of data with high Quality of Service requirements.
By using Virtual LANs (VLANs), a single physical LAN can be separated into multiple Virtual
LANs. VLANs can be used to:
■ Further improve LAN performance, as broadcast traffic is limited to LAN segments
serving members of the VLAN to which the sender belongs.
■ Provide security, as frames are forwarded to those stations belonging to the sender’s
VLAN, and not to stations in other VLANs on the same physical LAN.
■ Reduce the cost of moving or adding stations to function or security based LANs, as
this generally requires only a change in the VLAN configuration.