Download Where is the collision domain?

Chapter 15
Connecting LANs,
Backbone Networks,
and Virtual LANs
Pongpisit Wuttidittachotti
Faculty of Information Technology
King Mongkut's University of Technology North Bangkok
Email: [email protected], [email protected]
Line ID: pongpisitw
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Network Topologies
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A network’s geographical shape is referred to as the
network’s topology.
The five kinds of network topologies are:
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Bus topology
Ring topology
Star topology
Hybrid topology
Mesh topology
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Bus Topology
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The bus topology has a single cable, called the bus
or the trunk, to which every device on the network
connects.
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All of the messages on the bus pass by
each device, or node, on the bus.
It uses inexpensive coaxial cable that is
easy to install.
A break anywhere along the cable causes
all network traffic to stop.
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Ring Topology
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A ring topology forms when you connect a network’s nodes in
a circle.
 Messages flow in a single direction.
 A small packet called a token circulates the ring – a device
must have the token to use
the network.
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Star Topology
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In a star topology, each
device in the network connects
to
a central hub, which
distributes messages from one
node to another.
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Provides centralized control
Easy expansion
Fault tolerance
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Hybrid Topology
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A hybrid topology in a network employs more than one topology
to connect devices.
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Mesh Topology
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In a mesh topology,
multiple connections are
made.
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In mission-critical
operations, you want as
much redundancy as
possible so that
if one part of the network
goes down, the packets
can find an alternate path
to their destination.
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15.1 Connecting Devices
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Five connecting devices
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Repeaters
Hubs
Bridges
Switches
Routers
Gateway
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Figure 15.1 Five categories of connecting devices
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Repeaters
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A physical layer device the acts on bits not on frames or
packets
Can have two or more interfaces
When a bit (0,1) arrives, the repeater receives it and
regenerates it, the transmits it onto all other interfaces
Used in LAN to connect cable segments and extend the
maximum cable length  extending the geographical
LAN range
 Ethernet 10base5 – Max. segment length 500m – 4
repeaters (5 segments) are used to extend the cable to
2500m)
 Ethernet 10Base2- Max. segment length 185m - 4
repeaters (5 segments) are used to extend the cable to
925m
Repeaters do not implement any access method
 If any two nodes on any two connected segments
transmit at the same time collision will happen
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Figure 15.3 Function of a repeater
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Figure 15.2 A repeater connecting two segments of a LAN
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Hubs
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Acts on the physical layer
Operate on bits rather than frames
Also called multiport repeater
Used to connect stations adapters in a physical star topology but
logically bus
Connection to the hub consists of two pairs of twisted pair wire one
for transmission and the other for receiving.
Hub receives a bit from an adapter and sends it to all the other adapters
without implementing any access method.
does not do filtering (forward a frame into a specific destination or
drop it) just it copy the received frame onto all other links
The entire hub forms a single collision domain, and a single
Broadcast domain
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Collision domain: is that part of the network (set of NICs) when two or
more nodes transmit at the same time collision will happen.
Broadcast domain: is that part of the network (set of NIC) where each
NIC can 'see' other NICs' traffic broadcast messages.
Multiple Hubs can be used to extend the network length
For 10BaseT and 100BaseT the maximum length of the connection
between an adapter and the hub is 100 meters  the maximum length
between any two nodes is 200 m = maximum network length
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Ethernet
http://tjliu.myweb.hinet.net/COA_CH_12.htm
http://www.thefoa.org/tech/ref/appln/networks.html
http://www.automation-drive.com/bus-ethernet
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Figure 16.4
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Hubs
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Interconnecting with hubs
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Backbone hub interconnects LAN segments
Advantage:
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Extends max distance between nodes
Disadvantages
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Individual segment collision domains become one large collision
domain  (reduce the performance)
Can’t interconnect different Ethernet technologies(like 10BaseT &
100BaseT) because no buffering at the hub
Here we have a
single collision
domain and a
single
broadcast
domain
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Hubs Vs. Repeaters
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Hub are different than repeaters in the
following:
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The provide network management features by
gathering information about the network and
report them to a monitoring host connected to the
hub so some statistics about the network
(bandwidth usages, collision rates, average frame
sizes) can be generated.
If an adapter is not working the hub can disconnect
it internally and the network will not be affected.
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Bridges/switches
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Acts on the data link layer (MAC address level)
Used to divide (segment) the LAN into smaller LANs segments, or to connect
LANs that use identical physical and data link layers protocol (see figure in next
slide)
Each LAN segment is a separate collision domain
Bridge does not send the received frame to all other interfaces like hubs and
repeaters, but it performs filtering which means:
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Whether a frame should be forwarded to another interface that leads to the destination
or dropped
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This is done by a bridge table (forwarding table) that contains entries for the
nodes on the LAN
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The bridge table is initially empty and filled automatically by learning
from frames movements in the network
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An entry in the bridge table consists of : Node LAN (MAC) Address, Bridge
Interface to which the node is connected to, the record creation time
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A bridge runs CSMA/CD before sending a frame onto the link not like the hub
or repeater
Bridge frame handling is done in software
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Bridges
Connecting two or more LAN segments
together
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Bridges (Switches) Vs. Hubs
A Hub sending a packet form F to C.
A Switch sending a packet from F to C
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Figure 15.5 A bridge connecting two LANs
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Legacy Ethernet
Hub
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Switch
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Legacy Ethernet
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Half Duplex:
• One way traffic.
• Necessary on a shared media.
• Only one device can transmit at
a time.
• Collisions occur.
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Hub
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Legacy Ethernet
Switch
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Full Duplex:
• Two way traffic.
• Not a shared media.
• Dedicated switch
connection.
• A device can
transmit and
receive at the
same time.
• No Collisions.
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Ethernet
Hubs and Switches
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Hubs and Switches
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Collision Domain:
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The area of a network where collisions can occur.
Includes a hub and all connected devices.
Each port on a switch is considered a separate collision domain even
if there is only one device attached to the port.
Broadcast Domain:
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The area of a network where connected devices can receive a
broadcast.
Usually includes Layer 1 and 2 devices.
A collection of collision domains.
A router (Layer 3 device) is the usual boundary since routers block
broadcasts.
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Legacy Ethernet: Using Hubs
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Legacy Ethernet: Using Hubs
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So, what does a hub do
when it receives
information?
Remember, a hub is
nothing more than a
multiport repeater.
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Legacy Ethernet: Using Hubs
• The hub will flood it out
all ports except for the
incoming port.
• A hub is a Layer 1 device
and does NOT look at
Layer 2 addresses, so it is
fast in transmitting data.
• A hub or series of hubs is
called a single Collision
Domain.
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Legacy Ethernet: Using Hubs
•
Wasted
Bandwidth
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Disadvantages:
• Collision Domains
• Wasted Bandwidth
• All ports of the hub share the
total available bandwidth.
• Limited Scalability
• Increased Latency
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Legacy Ethernet: Using Hubs
Where is
the collision
domain?
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Legacy Ethernet: Using Hubs
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When a node wishes to communicate
with ALL hosts
on the network, it sends a BROADCAST
frame with a destination MAC address of
0xFFFFFFFFFFFF.
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All nodes on the network recognize that
they should look
at the contents of the frame.
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Broadcasts are used in the Address
Resolution Protocol (ARP), the Dynamic
Host Configuration Protocol (DHCP) and
other areas. (More Later!)
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Legacy Ethernet: Using Hubs
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Broadcasts cannot be entirely avoided if you are to have a dynamic
network.
• However, too many broadcasts can create a lot of unnecessary
traffic on a network and they should be minimized as much as
possible.
Layer 1 and Layer 2 devices (repeaters, hubs, bridges and switches)
must forward a broadcast.
• Layer 1 devices have no choice because they do not look at MAC
addresses.
• Layer 2 devices have no other choice because they cannot learn the
0xFFFFFFFFFFFF address.
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Legacy Ethernet: Using Hubs
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Host 1111 sends a broadcast to all
nodes on the network.
All hosts recognize the MAC
broadcast address and act on the
information in the frame.
•
Where is the Broadcast Domain?
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Ethernet: Using Switches
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Ethernet: Using Switches
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Switches are also known as
learning bridges or learning
switches.
A switch has a source
address table in cache
(RAM) where it stores
source MAC addresses for
each port.
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Ethernet: Using Switches
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Switch receives an Ethernet
frame.
Searches the source
address table for the
destination MAC address.
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Ethernet: Using Switches
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If it finds a match, it
forwards the frame by only
sending it out that port
(selective forwarding).
If the destination address
is not in the table, it floods
it out all ports.
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How does a switch learn an
address?
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• First, the switch will
see if the SA (1111) is
in it’s table.
• If it is, it resets a
timer.
• If it is NOT in the table
it adds it, with the
port number.
• Next the switch will
flood the frame out all
other ports, because
the DA is not in the
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source address
table.
How does a switch learn an
address?
• Most communications
involve some sort of
client-server relationship
or exchange of
information.
• Now 3333 sends data
back to 1111.
• The switch sees if it has
the SA stored. It does
NOT so it adds it.
• Next, it checks the DA
and sends it out port 1.
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How does a switch learn an
address?
• Now, with both MAC
addresses in the
table, any
information
between 1111 and
3333 can be sent
(selectively
forwarded) out the
appropriate port.
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Multiple Transmissions - No
Collisions
• Unlike a hub, a collision
does NOT occur, which
would cause the two PCs
to have to retransmit the
frames.
• The switch buffers the
frames and sends them
out port #6 one at a
time.
• The sending PCs have no
idea that there was
another PC wanting to
send to the same
destination.
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Collision Domains
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When there is only one
device on a switch port, the
collision domain is only
between the PC and the
switch.
With a full-duplex PC and
switch port, there will be
no collision, since the
devices and the medium
can send and receive at the
same time.
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Switches: Five Basic Operations
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Learning:
• Adds source MAC address/port to the table.
Aging:
• Addresses will be cleared after a specific length of inactivity.
Flooding:
• Sends a frame out all ports if the SA is not in the table.
Forwarding:
• Sends a frame out the proper port on a match.
Filtering:
• Will not forward a frame out the received port.
• Drops corrupt frames.
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Switch learning process
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When the switch receives a frame, it compares the source
address of the frame with each entry in the forwarding table
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If No match is found, the bridge will add to the table the frame source
address and the Interface on which the frame was received.
If a match is found, the bridge updates the Interface number on
which the frame was received if it is different from the one in the table
also it updates the record time
Then, the switch compares the destination address of the
frame with each entry in the forwarding table (MAC table)
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If a match is found then
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The bridge compares the interface number on which the frame was
received and the interface number in the table, if they are different the
bridge forwards the frame through the interface number stored in the table.
Otherwise, if they are the same the switches discards (drops) the frame.
If no match is found, the switch floods the frame on all interfaces
except the one on which the frame was received.
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Some switch features
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Implements CSMA/CD
switches Isolates collision domains (each LAN segment is a separate
collision domain), THIS WILL REDUCE THE POSSIBILITY OF
COLLISIONS AND result in higher total max throughput (see next
slide)
switch forwards a frame with broadcast address to all devices attached
to the whole network (single broadcast domain)
Can be used to combine Ethernet segments using different Ethernet
technologies (10Base2 and 100BaseT and 10BaseT) because it has
buffering capabilities
Increases reliability (how?), performance (how?), and security (how?)
Increases geographical coverage
 No limit on the size of the LANs connected through switches
Transparent: installing or removing a switch does not require the
stations networking software to be reconfigured.
(“plug-and-play”): no configuration necessary at installation of switch
/switch or when a host is removed from one of the LAN segments
Disadvantage: switch does not allow multiple paths between LAN
segments or between any two devices.
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Figure 13.14 Sharing bandwidth
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Collision domains in a nonbridged and bridged network
In heavy load, each station has an average
effective theoretical bandwidth = 10/12
Each station has an average effective
bandwidth equal =10/3
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Switch
Example:
Three LANs connected through a
bridge
Note: here we have three collision
domains and a single broadcast
domain
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Figure 16.8
Prior to spanning tree application
Switch
•When using switches, the network should
not contain any loop (there should be
exactly one path from any LAN to any
other LAN
•Loops can cause number of frames in the
LAN to increase indefinitely
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Effect of Loop of switches
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Figure 15.7 Loop problem in a learning switch
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Figure 15.10 Forwarding and blocking ports after using spanning
tree algorithm
 For any connected graph there is a spanning tree that
maintains connectivity but contains no closed loops
 Loops are logically disabled by the minimum spanning tree
algorithm
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Switches
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N-Port bridge where N is equal to number of stations
Usually used to connect individual computers not LANs like bridge
Allows more than one device connected to the switch directly to
transmit simultaneously
Can operates in Full-duplex mode (can send and receive frames at the
same time over the same interface)
Performs MAC address recognition and frame forwarding in
hardware (bridge in software)
Two types :
 Store-and-forward: switch receives the whole a frame on the input
line, buffers it briefly , performs error checking, then routes it to
the appropriate output line (similar to bridge). Buffering will cause
some delay.
 Cut-through: based on the fact that the destination address
appears at the beginning of the MAC frame, so once the address is
recognized the frame is directly sent to the appropriate output line
if the output buffer is empty (no need to buffer it).  no buffering
delay  NO ERROR CHECKING
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Isolated
collision
domains
Full-Duplex
operation
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Routers
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Operates at network layer = deals with packets not frames
Connect LANs and WANs with similar or different protocols
together
Switches and bridges isolate collision domains but forward
broadcast messages to all LANs connected to them. Routers
isolate both collision domains and broadcast domains
Acts like normal stations on a network, but have more than one
network address (an address to each connected network)
Deals with global address ( network layer address (IP)) not local
address (MAC address)
Routers Communicate with each other and exchange routing
information
Determine best route using routing algorithm by special software
installed on them
Forward traffic if information on destination is available
otherwise discard it (not like a switch or bridge)
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Figure 15.11 Routers connecting independent LANs and WANs
Routers
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An Institutional Network Using Hubs,
Ethernet Switches, and a Router
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switch
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switch
switch
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15.3 Virtual LANs
3 Collision domains
3 Broadcast domains
If we want to move computers from group1 to group3, then rewiring
(physical replacement) has to be done
What is the alternative solution??
VLAN: Virtual (logical) Local Area Network : Local
Area Network configured by software not by physical
wiring
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Figure 16.15 A switch using VLAN software
1
2
3
4
5
6
7
8
9
10
VLAN1: Ports 1,2,5,7
VLAN2: Ports 3,4,6
VLAN3: Ports 8,9,10
Separate broadcast domain 
separate network
Using the Virtual LAN technology will allow grouping computers
logically instead of physically.
VLAN divides the physical LAN into several Logical LANs
called VLANs
 Switch maintains a look up table to know to which LAN a
machine belongs to.
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Figure 15.17 Two switches in a backbone using VLAN software
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Note:
VLANs create broadcast domains.
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Advantages Of VLAN
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Reduce cost and installation time:
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Increase security:
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Instead of physically moving a station to another segment or another
switch, it can be moved by software.
A group of users needing a high security can be put into a VLAN so
that NO users outside the VLAN can communicate with them.
Stations belong to the same group can send broadcast messages that
will NOT be received by users in others VLAN groups
Creating Virtual Workgroups
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Stations located at physically different locations can be added easily to
the same broadcast domain so that they can send broadcast messages to
one another.
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EXAMPLE: people from different departments working on the same
project
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