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Chapter 15
Connecting LANs,
Backbone Networks,
and Virtual LANs
15.1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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.
Topics discussed in this section:
Passive Hubs
Active Hubs
Bridges
Two-Layer Switches
Routers
Three-Layer Switches
Gateways
15.2
Figure 15.1 Five categories of connecting devices
15.3
Figure 15.2 A repeater connecting two segments of a LAN
15.4
Note
A repeater connects segments of a LAN.
15.5
Note
A repeater forwards every frame;
it has no filtering capability.
15.6
Note
A repeater is a regenerator,
not an amplifier.
15.7
Figure 15.3 Function of a repeater
15.8
Figure 15.4 A hierarchy of hubs
15.9
Note
A bridge has a table used in
filtering decisions.
15.10
Figure 15.5 A bridge connecting two LANs
15.11
Note
A bridge does not change the physical
(MAC) addresses in a frame.
15.12
Figure 15.6 A learning bridge and the process of learning
15.13
Figure 15.7 Loop problem in a learning bridge
15.14
Figure 15.8 A system of connected LANs and its graph representation
15.15
Spanning Tree
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15.16
Spanning tree is a graph in which there is
no loop.
In bridged LAN spanning tree means
creating a topology in which each LAN can
be reached from any other LAN through
one path only (no loop).
Logical topology is created instead of
physical topology.
Spanning Tree
The process to find the spanning tree
involves three steps:
1- Select the root bridge (root of the tree),
based on the smallest router ID.
2- Find the shortest path from the root
bridge to every other bridge or LAN.
3- the combination of the shortest paths
creates the shortest tree.
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15.17
Figure 15.9 Finding the shortest paths and the spanning
tree in a system of bridges
15.18
Figure 15.10 Forwarding and blocking ports after using spanning
tree algorithm
15.19
Spanning Tree
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Based on the spanning tree ports marked
as:
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1 – Forwarding ports
2 – Blocking ports
* The process of creating and updating
spanning tree can be automated using
Dynamic Algorithm (software package).
15.20
Two-Layer Switches
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A two-layer switch is a bridge. A bridge
with many ports and a design that allows
better performance.
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15.21
A bridge with a few ports can connect a few
LANs together.
A bridge with many ports may be able to
allocate a unique port to each station (no
collision).
Routers
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15.22
A router is a three-layer device that routes
packets based on their logical addresses.
A router normally connects LANs and
WANs in the internet.
It has a routing table that is used to make
decisions about the route.
The routing table are normally dynamic
and are updated using routing protocols.
Figure 15.11 Routers connecting independent LANs and WANs
15.23
Three-Layer Switches
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15.24
A three-layer switch is a router, but a
faster and more sophisticated.
Three-layer switch provide faster table
lookup and forwarding.
The terms router and three-layer switch
are used interchangeably.
Gateway
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15.25
A gateway is normally a computer that
operates in all five layers of the Internet
or seven layers of OSI model.
A gateway takes an application message,
reads it, and interprets it, so it can be
used as a connecting device between two
internetworks that use different models.
Gateways can provide security.
In some textbooks the terms gateway and
router are used interchangeably.
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.
Topics discussed in this section:
Bus Backbone
Star Backbone
Connecting Remote LANs
15.26
Note
In a bus backbone, the topology
of the backbone is a bus.
15.27
Figure 15.12 Bus backbone
15.28
Note
In a star backbone, the topology of the
backbone is a star;
the backbone is just one switch.
15.29
Figure 15.13 Star backbone
15.30
Figure 15.14 Connecting remote LANs with bridges
15.31
Note
A point-to-point link acts as a LAN in a
remote backbone connected by
remote bridges.
15.32
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.
Topics discussed in this section:
Membership
Configuration
Communication between Switches
IEEE Standard
Advantages
15.33
LAN or VLAN
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15.34
A station is considered part of a LAN if it
physically belongs to that LAN.
The question is what if we need a virtual
connection between two stations
belonging to two different physical LANs?
Figure 15.15 A switch connecting three LANs
15.35
A switch connecting three LANs
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15.36
This is an example of switched LAN,
consists of three groups of users.
What if the network administrator needs
to move some of users from one group to
another, the solution is by rewiring.
Figure 15.16 A switch using VLAN software
15.37
A switch using VLAN software
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15.38
This is an example of the same switched LAN but divided
into three VLANs.
The idea of VLAN technology is to divide a LAN into
logical, instead of physical, segments.
Each VLAN is a work group in the organization. If a user
moves from one group to another, there is no need to
change the physical configuration.
The group membership in VLANs is defined by software,
not hardware.
All members belonging to a VLAN can be receive
broadcast messages sent to that particular VLAN.
Figure 15.17 Two switches in a backbone using VLAN software
15.39
Two switches in a backbone using VLAN software
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15.40
VLAN technology even allows the grouping
of stations connected to different switches
in VLAN.
This is an example of a good configuration
for a company with two separate
buildings. Each building can have its own
switched LAN connected by a backbone.
Note
VLANs create broadcast domains.
15.41
VLAN Membership
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What can be used to group stations in a
VLAN?
Venders use different characteristics such as port
numbers, MAC addresses, IP addresses, IP multicast
addresses, or a combination of two or more of these.
Port Numbers:
- switch port numbers can be used by administrators to define
which station is connected to which port number.
15.42
VLAN Configuration
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15.43
Stations are grouped into different VLANs. Stations are
configured in one of three ways: manual, automatic, and
semiautomatic.
In a manual configuration, the network administrator
uses the VLAN software to manually assign the stations
into different VLANs at setup.
In an automatic configuration, the stations are
automatically connected or disconnected from a VLAN
using criteria (e.g. project number) defined by the
administrator.