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Chapter 12

Connecting LANs and WANs:

Making Backbone Networks

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Differentiate between the different connecting devices and Differentiate between the different connecting devices and the layers each supports.the layers each supports.

Understand the functionality of a repeater and where it is used.Understand the functionality of a repeater and where it is used.

Understand the difference between a two-layer and a three-Understand the difference between a two-layer and a three-layer switch. layer switch.

After reading this chapter, the reader should After reading this chapter, the reader should be able to:be able to:

OOBJECTIVESBJECTIVES

Understand the functionality of a bridge and where it is used.Understand the functionality of a bridge and where it is used.

Understand the functionality of a router and where it is used.Understand the functionality of a router and where it is used.

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OOBJECTIVES (continued)BJECTIVES (continued)

Distinguish between a logical bus and logical star backbone.Distinguish between a logical bus and logical star backbone.

Distinguish between a bridge-based and a router-based back-Distinguish between a bridge-based and a router-based back-bone network.bone network.

Understand the functionality of a backbone network andUnderstand the functionality of a backbone network andwhere it is used.where it is used.

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CONNECTINGCONNECTINGDEVICESDEVICES

CONNECTINGCONNECTINGDEVICESDEVICES

12.112.1

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Figure 12-1

Connecting devices

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Figure 12-2

Repeater

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A repeater connects segments of aA repeater connects segments of aLAN together.LAN together.

Note:Note:

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A repeater forwards every packet;A repeater forwards every packet;it has no filtering capability.it has no filtering capability.

Note:Note:

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Technical Focus:Technical Focus: Repeaters and AmplifiersRepeaters and Amplifiers

It is tempting to compare a repeater to an amplifier, but the comparison is inaccurate. An amplifier cannot discriminate between the intended signal and noise; it amplifies equally everything fed into it. A repeater does not amplify the signal; it regenerates it. When it receives a weakened or corrupted signal, it creates a copy bit for bit, at the original strength. A repeater is a regenerator, not an amplifier.

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Figure 12-3

Hubs

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Technical Focus:Technical Focus: Active versus Passive HubActive versus Passive Hub

A passive hub is a hub that is used to create a connection; it does not regenerate signals. An active hub, on the other hand, is a connector as well as a repeater. Today, most hubs are active hubs.

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A bridge has a table used inA bridge has a table used infiltering decisions. filtering decisions.

Note:Note:

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Figure 12-4

Bridge

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A bridge connects segments of aA bridge connects segments of aLAN together. LAN together.

Note:Note:

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A router is a three-layerA router is a three-layer(physical, data link, and network)(physical, data link, and network)

device. device.

Note:Note:

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A repeater or a bridge connects A repeater or a bridge connects segments of LAN together.segments of LAN together.

A router connects independent LANs A router connects independent LANs or WANsor WANs

to create an internetwork (internet).to create an internetwork (internet).

Note:Note:

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Figure 12-5

Routing example

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A router changes the physical A router changes the physical addresses in a packet. addresses in a packet.

Note:Note:

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Technical Focus:Technical Focus: Multiprotocol RouterMultiprotocol Router

At the network layer, a router by default is a single-protocol device. In other words, if two LANs are to be connected by a router, they must use the same protocol at the network layer. The reason behind this is that the routing table must use one single addressing format.

However, multiprotocol routers have been designed to route packets belonging to two or more protocols. For example, a two-protocol router (for example, IP and IPX) can handle packets belonging to either of the two protocols. It can receive, process, and send a packet using the IP protocol or it can receive, process, and send a packet using the IPX protocol. In this case, the router has two routing tables: one for IP and one for IPX.

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Technical Focus:Technical Focus: BroutersBrouters

A brouter (bridge/router) is a single-protocol or multiprotocol router that sometimes acts as a router and sometimes as a bridge. When a single-protocol brouter receives a packet belonging to the protocol for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address.

Likewise, when a multiprotocol brouter receives a packet belonging to one of the protocols for which it is designed, it routes the packet based on the network layer address; otherwise, it acts as a bridge and passes the packet using the data link layer address.

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Technical Focus:Technical Focus: GatewaysGateways

Today, the term gateway is used mostly as a synonym for a router. However, in the past, the two terms had different meanings. A gateway defined a device that could potentially operate in all five layers of the Internet model. It was a protocol converter. A router was a device that could transfer, accept, and relay packets only across networks using similar protocols. A gateway, on the other hand, could accept a packet formatted for one protocol (e.g., AppleTalk) and convert it to a packet formatted for another protocol (e.g., TCP/IP) before forwarding it.

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BACKBONEBACKBONENETWORKSNETWORKSBACKBONEBACKBONENETWORKSNETWORKS

12.212.2

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In a logical bus backbone, the logicalIn a logical bus backbone, the logicaltopology of the backbone is a bus;topology of the backbone is a bus;

the physical topology can be the physical topology can be a bus or a star. a bus or a star.

Note:Note:

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Figure 12-6

Bridged-base logical bus network

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Figure 12-7

Router-based logical bus backbone

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In a logical star backbone, the logicalIn a logical star backbone, the logicaltopology of the backbone is a star;topology of the backbone is a star;

the backbone is just one switch.the backbone is just one switch.

Note:Note:

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Figure 12-8

Logical star backbone

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Technical Focus:Technical Focus: FDDIFDDI

Fiber distributed data interface (FDDI) is a local area network protocol standardized by ANSI and the ITU-T. It supports data rates of 100 Mbps. FDDI is implemented as a dual ring as shown below.