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15.1
Chapter 15
Connecting LANs, Backbone Networks,
and Virtual LANs
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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15.2
15-1 CONNECTING DEVICES15-1 CONNECTING DEVICES
In this section, we divide connecting devices into five In this section, we divide connecting devices into five different categories based on the layer in which they different categories based on the layer in which they operate in a network.operate in a network.
Passive HubsActive HubsBridgesTwo-Layer SwitchesRoutersThree-Layer SwitchesGateways
Topics discussed in this section:Topics discussed in this section:
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15.3
Figure 15.1 Five categories of connecting devices
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15.4
Figure 15.2 A repeater connecting two segments of a LAN
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15.5
A repeater connects segments of a LAN.
Note
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15.6
A repeater forwards every frame; it has no filtering capability.
Note
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15.7
A repeater is a regenerator, not an amplifier.
(same signal strength, bit for bit copy)
Note
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15.8
Figure 15.3 Function of a repeater
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15.9
Figure 15.4 A hierarchy of hubs
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15.10
A bridge checks MAC addresses. It has a table used in filtering decisions
(Forward? Drop?). The table shows the map between MAC addresses and ports.
Note
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15.11
Figure 15.5 A bridge connecting two LANs
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15.12
A bridge does not change the physical (MAC) addresses in a frame.
Note
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15.13
Figure 15.6 A learning bridge and the process of learning
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15.14
Figure 15.7 Loop problem in a learning bridge
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15.15
Use spanning three in graph theory to avoid loop topology. Spanning tree is a
graph without loop.
Note
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15.16
Figure 15.11 Routers (three-layer switch) connecting independent LANs and WANs
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15.17
15-2 BACKBONE NETWORKS15-2 BACKBONE NETWORKS
A backbone network allows several LANs to be A backbone network allows several LANs to be connected. In a backbone network, no station is connected. In a backbone network, no station is directly connected to the backbone; the stations are directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs. part of a LAN, and the backbone connects the LANs.
Bus BackboneStar BackboneConnecting Remote LANs
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15.18
In a bus backbone, the topology of the backbone is a bus.
Note
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15.19
Figure 15.12 Bus backbone
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15.20
In a star backbone, the topology of the backbone is a star;
the backbone is just one switch.
Note
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15.21
Figure 15.13 Star backbone
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15.22
Figure 15.14 Connecting remote LANs with bridges
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15.23
15-3 VIRTUAL LANs15-3 VIRTUAL LANs
We can roughly define a We can roughly define a virtual local area networkvirtual local area network (VLAN) as a local area network configured by (VLAN) as a local area network configured by software, not by physical wiring.software, not by physical wiring.
MembershipConfigurationCommunication between SwitchesIEEE StandardAdvantages
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15.24
Figure 15.15 A switch connecting three LANs
Physical wiring makes it hard to dynamically change group allocation
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15.25
Figure 15.16 A switch using VLAN software
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15.26
Figure 15.17 Two switches in a backbone using VLAN software
Good for a company with two separate buildings
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15.27
VLANs create broadcast domains.
Note
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VLAN: Membership
Switch port Numbers MAC Address IP Address Multicast IP Address Combination
15.28
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VLAN Configuration
Manually Automatic Semiautomatic
15.29
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VLAN: Communication between switches Table maintenance Frame tagging
Extra header added to MAC frame Time-Division Multiplexing (TDM)
15.30
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VLAN: Advantages
Cost and time reduction Creating Virtual Work Groups Security
Separation of broadcast messages
15.31