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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
5 categories of connecting devices
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Defined as:
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15.4
Those which operate below the physical layer such as
a passive hub.
Those which operate at the physical layer such as the
repeater or an active hub.
Those which operate at the physical and data link
layers such as bridge or two-layer switch.
Those which operate at the physical, data link and
network layers such as the router or three-layer
switch.
Those which can operate at all five layers such as the
gateway.
15.5
Passive Hub
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15.6
Connects wires coming from different
branches.
Location on the Internet model – below
the physical layer.
Considered part of the transmission
media.
Repeater
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15.7
Repeater is a device that operates only in
the physical layer.
Signals can carry information for a limited
distance.
Weak signals are regenerated and sent
out to the next hop.
Increases the distance of your network.
Repeater
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15.8
Is used to extend the physical length of a
network.
Regenerates the electrical signal and then send
the refreshed signal.
Repeaters do not have the intelligence to route
traffic.
Repeaters cannot connect two LANs; it connects
two segments of the same LAN.
Segment: creating physical network. In another
words, it is a subset of larger network.
Figure 15.2 A repeater connecting two segments of a LAN
15.9
Note
A repeater forwards every frame;
it has no filtering capability.
15.10
Note
A repeater is a regenerator,
not an amplifier.
15.11
Repeater
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Not an amplifier
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15.12
Amplifier does not discriminate between the
intended signal and noise.
Repeater – regenerates bit by bit a
weakened signal.
Figure 15.3 Function of a repeater
15.13
Active Hubs
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15.14
Multiport repeater
Used to create connections between
stations in a physical star topology.
Does not have a filtering capability;
Does not have the intelligence to find from
which port the frame should be sent out.
It regenerates the corrupted signals and
send them out from every port.
Figure 15.4 A hierarchy of hubs
15.15
Bridge
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15.16
Operates in both the Physical and Data
Link Layer
Allows you divide your network into
smaller segments
Contain logic that allows them to keep
traffic for each segment separate.
Check the source and destination MAC
addresses in the frame.
Also regenerates the signal to eliminate
attenuation
Bridges
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15.17
Bridges are used to connect 2 or more
LANs.
Can connect similar LANs (2 Ethernet
LANs)
Can also connect different LANs (Token
Ring to a Ethernet).
Cannot maintain the data integrity.
Note
A bridge has a table used in
filtering decisions.
15.18
Figure 15.5 A bridge connecting two LANs
15.19
Note
A bridge does not change the physical
(MAC) addresses in a frame.
15.20
Types of Bridges
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15.21
Simple Bridge
Multiport Bridge
Transparent Bridge
Simple Bridge
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15.22
Uses a look up table that contains the
physical addresses of every station
connected to it. The table indicates which
segment the station belongs.
Addresses must be entered manually (by
hand).
maintenance is time consuming
Least expensive hardware
Multiport Bridge
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15.23
Used to connect more than 2 LANs
together.
Uses multiple tables to hold the physical
addresses of each LAN.
Transparent Bridge
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15.24
Sometimes called a Learning Bridge
Does not require you to manually input
the physical addresses of your stations.
Automatically creates and updates the
Look up table.
Uses the source and destination addresses
in each packet.
Transparent Bridge
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According to IEEE 802.1d specfications, a
system equipped with transparent bridges
must meet 3 criteria:
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15.25
Frames must be forwarded from one station
to another.
The forwarding table is automatically made by
learning frame movements in the network.
Loops in the system must be prevented.
Forwarding
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15.26
A transparent bridge must correctly
forward the frames using its table.
Learning
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Requires a table that dynamically maps
addresses to ports automatically.
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15.27
Bridge inspects both the destination and the
source address.
The destination address is used for the
forwarding decision (table lookup);
The source address is used for adding entries
to the table and for updating purposes.
Example of a Learning Bridge
15.28
Loop Problem
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15.29
If redundant bridges in the system exist,
the frames get forwarded twice to the
receiver. Does not fill in the destination
address correctly.
Floods the network.
TO solve the problem IEEE requires that
bridges use the spanning tree algorithm
Loop Problem in a Learning Bridge
15.30
Spanning tree
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Is a graph in which there is no loop. Creating a topology
in which each LAN can be reached from any other LAN
through one path only (no loop).
To find the spanning tree:
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The algorithm finds the shortest path (lowest cost) from
the root bridge to every other bridge or LAN.
The ports on each bridge as designated as Forwarding
ports others are blocking ports
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15.31
Every bridge has a built in ID
ID is broadcasted to all other bridges.
The smallest ID is selected as the root bridge.
Forwarding ports: is a part of the spanning tree and a frame can be
forwarded.
Blocking ports: is not a part of the spanning tree and a frame cannot be
forwarded.
Source Routing Bridges
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15.32
TO prevent loop problems, source routing
bridges require that the sending station
defines the bridges that the frame must
visit.
The sending stations defines the
addresses of these bridges and they are
included in the frame.
Source address, Destination address and
all addresses of bridges
Source Routing Bridges
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15.33
Designed by IEEE to be used with Token
Ring LANs
Token Ring LANs are not very common
today.
Layer 2 and 3 switches
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15.34
Layer 2 switch functions as a device that
operates at the Physical and Data Link layer.
Uses Mac addressing
Same as a Bridge, but with many ports and
better (faster) performance.
Has a filtering capability based on the MAC
address.
Some are more sophisticated and include a
buffer that to hold frames for processing.
New two layer switch is called Cut-through
switch forwards the frame as soon as they check
the MAC addresses in the header of the frame.
3 Switches/Routers
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15.35
Layer 3 switch is the same as a router.
Routes traffic based on logical address
such as IP address.
Router connects LANS and WANs.
Addressing tables are dynamic and are
updated using routing protocols.
The switching fabric in a three-layer
switch allows faster table lookup and
forwarding.
A Router
15.36
Router with LAN and WAN
15.37
Internetworking devices
Routers
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15.38
More sophisticated hardware than the
Networking devices
Used to connect 2 or more networks.
Using addresses, it can determine several
possible paths to route packets.
Uses Logical Address – IP address
Determines the best path.
Internetworking devices
Routers
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15.39
Whenever there are multiple paths from
Source to Destination, the Router
determines the best path.
Internetworking devices
Gateway
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15.40
Used as a Protocol Converter
Gateways are usually routers with
additional software installed.
Usually a computer that operates in
all OSI layers.
Also able to provide some security by
filtering unwanted application-layer
messages.
A Gateway
15.41
Figure 21-13
A Gateway
15.42
Internetworking devices
Gateway
Allows you to connect two
different networks using different
protocols to each other.
 Example : connect an AppleTalk
network to a TCP/IP network –
must use a Gateway.
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15.43
Backbone Networks
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15.44
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: is used as a distribution
backbone to connect different building in an
organization.
Star Backbone: the backbone is just one
switch that connect the LANs. It is used as a
distribution backbone inside a building.
Bus Backbone
15.45
Star Backbone
15.46
Virtual LANs - VLANs
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15.47
Local area Network configured by
software, not by physical wiring.
Divides a LAN into logical , instead of
physical segments.
A switch using VLAN Software
15.48
Advantages of VLAN
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15.49
VLANs can reduce the relocation cost of
stations going from one group to another.
Physical reconfiguration takes times and
costly.
Can be used to create virtual work groups.
Extra measure of security. People
belonging to the same group can send
broadcast messages with the guaranteed
assurance of users in other groups will not
receive these messages.