Download ccna3 3.0-04 Introduction to LAN Switching

Module 4
Introduction to LAN Switching
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Objectives
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LAN congestion and its effect on network
performance
Advantages of LAN segmentation in a network
Advantages and disadvantages of using bridges,
switches, and routers for LAN segmentation
Effects of switching, bridging, and routing on
network throughput Fast Ethernet technology and
its benefits
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CSMA/CD prevents multiple devices from transmitting
at the same time.
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The Ethernet/802.3 Interface
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Ethernet is known as a shared-medium
technology – all the devices are connected to
the same delivery media.
Ethernet media uses a data frame broadcast
method of transmitting and receiving data to all
nodes on the shared media.
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Standard Ethernet using Carrier Sense
Multiple Access/ Collision Detection
(CSMA/CD) and a shared medium can
support data transmission rates of up to 10
Gbps (gigabits per second).
Goal of Standard Ethernet is to provide a best
effort delivery service and allow all devices on
the shared medium to transmit on an equal
basis.
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Performance of a shared media Ethernet/802.3
LAN can be negatively effected by several factors.
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The data frame broadcast delivery nature of
Ethernet/802.3 LANs
CSMA/CD access methods allow only one station to
transmit at a time.
Network congestion due to increased bandwidth
demands from multimedia applications such as video
and the Internet.
Normal latency (propagation delay) of frames as they
travel across the LAN layer 1 media and pass through
layer 1, 2 and 3 networking devices.
Extending the distances of the Ethernet/802.3 LANs
using Layer 1 repeaters.
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Half-Duplex Design
Tx
Ethernet
Controller
Collision
Detection
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Tx
Loopback
Loopback
Rx
Ethernet NIC
Transmit
Receive
Collision
Detection
Ethernet
Controller
Rx
Ethernet NIC
Ethernet physical connector provides several circuits
Most important are TX (transmit), RX (receive), and CD
(collision detection)
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Half-Duplex Ethernet Design
(Standard Ethernet)
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The most important of these circuits are the
receive (RX), transmit (TX), and CD (collision
detection0.
The transmit (TX) circuit is active at the
transmitting station.
The receive (RX) circuit is active at the
receiving station.
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To the network this appears as a single one
way bridge.
Both devices are contending for the right to
use the single shared medium.
The CD (collision detection) circuit on each
node contends for the use of the network
when the two nodes attempt to transmit at
the same time.
After a collision occurs, the hosts will
resume transmitting based on the hold
time calculated by a back-off algorithm.
Then the host will determine if the
network is clear before attempting to
retransmit.
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Congestion and Bandwidth
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To relieve network congestion more
bandwidth is needed or the available
bandwidth must be used more
efficiently.
“Throwing bandwidth at the problem”.
This could be attacking the symptom and
not the problem.
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Propagation Delay
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Latency is also known as Propagation delay is
the time a frame or packet requires to travel
from the source to destination on the
network.
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The greater the number of devices the
greater the latency or propagation delay
adding hosts simply increases collisions,
increases jam signals decreasing
throughput .
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Ethernet Transmission Times
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Each Ethernet bit uses a 100ns window for
transmission.
A byte is equal to eight bits.
Therefore, one byte takes a minimum of 800ns
to transmit (8 bits at 100ns = 800ns).
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A 64 byte frame requires 51,200ns or 51.2
microseconds to transmit
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1 Ethernet bit uses a 100ns window for
transmission.
A byte is equal to eight bits.
Therefore, one byte takes a minimum of 800ns to
transmit (8 bits at 100ns = 800ns).
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1 microsecond = 1000 nanoseconds
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1 byte = 8 bits
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(64 bytes at 800ns) = 51,200ns
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(51,200ns/1000) =
51.2 microseconds).
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Extending Shared Media LANs
using Repeaters
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Signal attenuation –Signal weakens as they
travels through the network due to resistance in
the medium.
A repeater is used to extend the geography of
a LAN allowing more users to share that same
network.
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Improving LAN Performance
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The performance of a network can be
improved in a shared media LAN by:
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Segmenting the network using bridges,
routers, or switches
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Using full duplex transmitting
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Upgrade to a faster Ethernet standard
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Segment LANs?
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Each segment uses the (CSMA/CD) protocol
to manages traffic on the segment.
By segmenting a network - less devices are
sharing the same bandwidth
Each segment is its own collision domain.
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Segmented LANs?
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In a segmented Ethernet LAN messages
passed between segments is transmitted on a
network backbone using a bridge, switch, or
router.
The backbone network is its own collision
domain and uses CSMA/CD to manage between
segments.
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Segmentation with Bridges
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Bridges are Layer 2 devices, independent of
Layer 3 protocols used by routers
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they transmit data frames regardless of
which Layer 3 protocol is being used
They are transparent to the other devices
on the network.
Bridges increase latency (delay) in a
network by 10-30%.
Why?
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A bridge is by default a store and forward device
It examines the destination MAC address to determine
through which interface the frame will be forward.
 If there is no match in the CAM table, the frame is
flooded out all other interfaces
Bridges “learn” network segments by building an address
table, a CAM (Content Address Memory), containing the
(MAC) address of each network device that accesses the
bridge and pairs it with its network segment.
 Collision domains are created, not broadcast domains.
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Segmentation using Routers
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Routers operate at network layer and base
routing decisions on the Layer 3 IP protocol
address.
Routers perform higher level functions than
do bridges consequently they operate at a
higher latency.
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Routers?
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Segment broadcast domains
Forward packets based on destination network
layer addresses, i.e. IP
Segment collision domains
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More collision domains,
but more bandwidth
for each user
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Segmentation with LAN Switches
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A switch segments a LAN into microsegments
creating collision free domains from one larger
collision domain, not broadcast domains.
Switched Ethernet available bandwidth can
reach close to 100%.
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LAN Switch Latency
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Each switch on an Ethernet LAN adds latency
to the network.
The type of switching used can help overcome
the built in latency of some switches.
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Full-Duplex Ethernet Overview
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Full duplex Ethernet allows the transmission of a
packet and the reception of a packet at the
same time.
Requires two pairs of conductors and a switched
connection between each node
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Simultaneous transmission and reception of
frames is called bidirectional traffic (two-way)
and on a 10Mbps circuit yields 20Mbps of
throughput.
The network interface cards (NICs) on
both ends of the circuit require full duplex
capabilities.
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Full-Duplex Ethernet Design
TX
Tx
Full
Duplex
Ethernet
Controller
Collision
Detection
Loopback
Rx
RX
Tx
Loopback
Collision
Detection
Rx
Full
Duplex
Ethernet
Controller
• Transmit circuit connects directly to receive circuit
• No collisions
• Significant performance improvement
• Eliminates contention on Ethernet point-to-point links
• Uses a single port for each full-duplex
connection
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Using Full Duplex
Half Duplex
Full Duplex
HUB
• Nodes must
– Be directly attached to a dedicated
switched port
– Have installed network interface card that
supports full duplex
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Full-Duplex Ethernet Design
Standard Ethernet normally can only use 5060% of the 100Mbps available bandwidth.
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This is due to collisions and latency.
Full duplex Ethernet offers 100% of the
bandwidth in both directions.
This produces a potential 200Mbps
throughput – 100Mbps TX and 100Mbps
RX.
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This virtual network circuit exists only when
two nodes need to communicate this circuit is
established within the switch.
It Allows multiple users to communicate in
parallel via these virtual circuits.
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Source MAC
address is
used to build
this table
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How a LAN Switch Learns Addresses
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When messages are received by the switch
their:
 addresses are read and stored in the CAM
(Content Address Memory).
Each time an address is stored it is time
stamped.
 This allows addresses to be stored for a set
period of time.
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But more domains
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Benefits of Switching
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A LAN switch allows many users to
communicate in parallel :
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through the use of virtual circuits
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dedicated network segments
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in a collision free environment.
Very cost effective.
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Symmetric Switching
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A symmetric switch is optimized through even
distribution of network traffic across the entire
network .
All networks using the same bandwidth.
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before forwarding
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Asymmetric Switching
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Asymmetric switching is best exemplified in
client-server network traffic flows where multiple
clients are simultaneously communicating with a
server.
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Each usually at a lower bandwidth than the
server
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Memory Buffering
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The area of memory where the switch stores the
destination and transmission data is called the
memory buffer.
This memory buffer can make use of two
methods for forwarding packets:
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port based memory buffering
shared memory buffering.
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Port based memory buffering packets are
stored in queues that are linked to specific
incoming ports.
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Problem: One port may fill while another is
empty.
Shared memory buffering deposits all
packets into a common memory buffer that is
shared by all the ports on the switch.
(Better!)
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3 frame transmission modes in a
switch (+one variation)
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Three Switching Methods
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Store and Forward - the entire frame is
received before any forwarding takes place.
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Latency occurs while the frame is being
received; the latency is greater with larger
frames because the entire frame takes
longer to read.
Error detection is high because of the
time available to the switch to check for
errors while waiting for the entire frame to
be received.
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Cut-through the switch reads the
destination address before receiving the
entire frame.
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The frame is then forwarded before the
entire frame arrives.
This mode decreases the latency of the
transmission and has poor error detection.
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Fragment-Free Switching
The switch reads only the
1st 64 bytes of the
incoming frame and then
forwards the frame to its
destination port
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Means the
switch is in cut
through mode
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Adaptive Cut Through
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Combines cut through with store and
forward
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The switch uses cut-through until there are a
given number of errors
Then the switch will change to store and
forward method
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Emerging Trends:
The Network Evolution
Shared to Switched
The
New
Wiring
Closet
HUB
The
Old
Wiring
Closet
HUB
VLAN
System
HUB
HUB
LAN
Campus
Switch
HUB
The New Backbone
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Benefits of Switches
• Number of collisions reduced
• Simultaneous, multiple communications
• High-speed uplinks
• Improved network response
• Increased user productivity
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Module 4
Switching Concepts
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