15.1
Chapter 15
Connecting LANs, �Backbone Networks, �and Virtual LANs
15.2
15-1 CONNECTING DEVICES
In this section, we divide connecting devices into five different categories based on the layer in which they operate in a network.
Passive Hubs
Active Hubs�Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways
Topics discussed in this section:
15.3
Figure 15.1 Five categories of connecting devices
15.4
Figure 15.2 A repeater connecting two segments of a LAN
15.5
A repeater connects segments of a LAN.
Note
15.6
A repeater forwards every frame; �it has no filtering capability.
Note
15.7
A repeater is a regenerator, �not an amplifier.
Note
15.8
Figure 15.3 Function of a repeater
15.9
Figure 15.4 A hierarchy of hubs
15.10
A bridge has a table used in �filtering decisions.
Note
15.11
Figure 15.5 A bridge connecting two LANs
15.12
A bridge does not change the physical (MAC) addresses in a frame.
Note
15.13
Figure 15.6 A learning bridge and the process of learning
15.14
Figure 15.7 Loop problem in a learning bridge
15.15
Figure 15.8 A system of connected LANs and its graph representation
15.16
Figure 15.9 Finding the shortest paths and the spanning � tree in a system of bridges
15.17
Figure 15.10 Forwarding and blocking ports after using spanning � tree algorithm
15.18
Figure 15.11 Routers connecting independent LANs and WANs
15.19
15-2 BACKBONE NETWORKS
A backbone network allows several LANs to be connected. In a backbone network, no station is directly connected to the backbone; the stations are part of a LAN, and the backbone connects the LANs.
Bus Backbone�Star Backbone�Connecting Remote LANs
Topics discussed in this section:
15.20
In a bus backbone, the topology �of the backbone is a bus.
Note
15.21
Figure 15.12 Bus backbone
15.22
In a star backbone, the topology of the backbone is a star;
the backbone is just one switch.
Note
15.23
Figure 15.13 Star backbone
15.24
Figure 15.14 Connecting remote LANs with bridges
15.25
A point-to-point link acts as a LAN in a remote backbone connected by �remote bridges.
Note
15.26
15-3 VIRTUAL LANs
We can roughly define a virtual local area network (VLAN) as a local area network configured by software, not by physical wiring.
Membership�Configuration
Communication between Switches
IEEE Standard
Advantages
Topics discussed in this section:
15.27
Figure 15.15 A switch connecting three LANs
15.28
Figure 15.16 A switch using VLAN software
15.29
Figure 15.17 Two switches in a backbone using VLAN software
15.30
VLANs create broadcast domains.
Note