Chapter 14. Switching Introduction; Introduction - Allied Telesis SwitchBlade x908 Series Software Reference Manual

Switching Introduction

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 and all
destined for the same output port.
The switch can be used to:
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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.
Software Reference for SwitchBlade® x908, x900 and x600 Series Switches
TM
AlliedWare Plus Operating System - Software Version 5.3.1 C613-50007-01 REV B