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Switching on the AR440S, AR441S and AR450S 3-3

Software Release 2.6.6

C613-03079-00 REV A

Introduction

This chapter gives an overview of Layer 1 (physical layer) and Layer 2 (data

link layer) switching, describes the support for switching, and how to

configure and operate the switch ports on the router.

The router 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.

The router 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. By

default, all switch ports on the router are included in the same VLAN.

Access to the physical link may not always be instant, so the router must be

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

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

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

forward them based on knowledge of which network contains the station with

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

acts as an intelligent filtering device, redirecting or blocking the movement of

frames between networks.

Because frames may be received faster than they can be forwarded, there are

Quality of Service queues in which frames await transmission according to their

priority.

The router can:

■ 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 router 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

router 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 which have a common data link layer protocol but different

physical media, for example, Ethernet 10BASET, 100BASET and 10BASEF.

■ Prioritise 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 only 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-based or

security-based LANs, as this generally requires a change in the VLAN

configuration.

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