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3-4 AT-8600 Series Software Reference

Software Release 2.6.4

C613-03081-00 REV A

Introduction

This chapter gives an overview of Layer 1 (the physical layer), 2 (the data link

layer), and 3 (the network layer) switching, and describes the support for

switching and how to configure and operate the switching functions.

The switch, also referred to as a MAC (media access control) bridge, a data link

relay or a level 2 relay, can 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. It 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 stations in other VLANs.

The switch operates at the data link layer, transparent to higher layer protocols,

transferring frames between the data link layers of the networks to which it is

attached. A bridge accesses each physical link according to the rules for that

particular network. Access may not always be instant, so a bridge must be

capable of storing and forwarding frames. Since the switch can store and

forward frames, it can examine and discard or admit frames according to their

VLAN tag fields. The switch can also examine the address fields of the frames

and forward the frames based on knowledge of which network contains the

station with an address matching the frame’s destination address. In this way,

the switch can act as an intelligent filtering device, redirecting or blocking the

movement of frames between networks.

Because the switch may receive frames faster than it can forward them, the

switch has Quality of Service queues in which frames await transmission

according to their priority.

The switch can be used to:

■ Increase the physical extent and/or 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 the

frame on another LAN, using the normal access methods for each LAN. 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 together to form

an extended LAN capable of supporting more stations than either of the

individual LANs.

■ 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.

■ Prioritise the transmission of data with high Quality of Service

requirements.

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