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Chapter 16 Chapter 16 Connecting LANs, Backbone Connecting LANs, Backbone Networks, and Virtual LANsNetworks, and Virtual LANs

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16.1 Connecting Devices16.1 Connecting Devices

Repeaters

Hubs

Bridges

Two-Layer Switches

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Connecting DevicesConnecting Devices

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RepeaterRepeater

• Repeater only operates in the physical layer• Repeater regenerates the signal• Doesn’t connect two LANs, connects two segments of the same LAN

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RepeaterRepeater

A repeater connects segments of a LAN.

A repeater forwards every frame; it has no filtering capability

A repeater is a regenerator, not an amplifier.

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Function of RepeaterFunction of Repeater

• Must be placed so that a signal reaches it before noise changes

the meaning of its bits

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HubsHubs

• Hub is a multiport repeater

• Creates connections between stations in a physical star topology

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BridgeBridge

Bridge operates in both the physical and the data link layers

As a physical layer device, it regenerates the signal

As a data link layer device, it checks the physical (MAC) addresses

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BridgeBridge

A bridge has a table used in filtering decisions.

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BridgeBridge

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BridgeBridge

A bridge does not change the physical (MAC) addresses in a frame.

Transparant BridgeBridge in which stations are completely unaware of the bridge’

s existence

System equipped with transparent bridges must meet three criteria (IEEE 802.1d):

Frames must be forwarded from one station to another Forwarding table is automatically made by learning from move

ments Loops must be prevented

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Learning BridgesLearning Bridges

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Loop ProblemLoop Problem

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Prior to spanning tree application

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Applying spanning tree

• Bridge with smallest ID is the root bridge• Mark one port of each bridge as the root port – port with the least-cost path from the bridge to the root bridge• Choose a designated bridge for each LAN – has the least-cost path between the LAN and the root bridge – make the corresponding port the designated port • Mark the root port and designated port as forwarding ports, the others as blocking ports

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Forwarding ports and blocking ports

•Dynamic algorithm – spanning tree algorithm is done dynamically with software in the bridge

using Bridge Protocol Data Unit (BPDU)

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Bridges Connecting Different LANs

Bridge should be able to connect LANs using different protocols, issues to be considered: Frame format – Ethernet vs. wireless frame Max data size – frames too large must be fragmented into

several frames, no protocol at the data link layer allows for fragmentation and reassembly of frames

Data rate – each LAN has its own data rate Bit order – some send most significant bit first, some send

least significant first Security – wireless has security measures at the dl layer,

Ethernet does not Multimedia support – some support, some do not

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16.2 Backbone Networks16.2 Backbone Networks

Bus Backbone

Star Backbone

Connecting Remote LANs

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Bus BackboneBus Backbone In a bus backbone, the topology of the backbone is a bus.

Normally used to connect different buildings in an organization

Bridge blocks frames sent internal to the LAN

Backbone receives frame if going from one LAN to another

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Star BackboneStar Backbone In a star backbone, the topology of the backbone is a star; the

backbone is just one switch.

• Used as distribution backbone inside a building

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Connecting remote LANs

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Connecting remote LANs

A point-to-point link acts as a LAN in a remote backbone connected by remote bridges.

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Virtual LANsVirtual LANs

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A switch using VLAN software

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Two switches in a backbone using VLAN software

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VLANVLAN

VLANs create broadcast domains.

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Questions !Questions !