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CCNA3: Switching Basics and Intermediate Routing v3.0

CISCO NETWORKING ACADEMY PROGRAM

Switching Concepts

Introduction to Ethernet/802.3 LANs

Introduction to LAN SwitchingSwitch Operation

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Chapter 4 Switching Concepts

Ethernet Technology Overview Ethernet multi-access

broadcast technology

Uses CSMA/CD

Collisions impact on

network performance

Layer 2 devices can

improve performance

Media includes CAT5(e),

fibre, wireless

Speeds from 10Mbps to

10,000 Mbps

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Network Growth

Bandwidth needs have increased

Internet/intranet/email

Multimedia Increasing use of enterprise servers

Ethernet has developed to meet challenge

10Mbps, 100Mbps, 1000Mbps, 10Gbit

Coaxial, Twisted Pair, Fibre Optic, Wireless

Repeaters, hubs, bridges, switches, routers

BUT you must understand the features of all this

technology to gain best performance in your network

design!

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Hubs Layer 1 devices

Regenerate, retime,

amplify signals 1 collision/bandwidthdomain

Broadcasts propagatedout of every port

Only 1 device cantransmit at a time

Only 50-60% bandwidthavailable

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Bridges Layer 2 device

Splits network into 2

collision/bandwidthdomains

Broadcasts are forwarded

Local traffic stays local

Checks Layer 2 MACaddresses in 802.3 frame

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Switches Layer 2 device

Learns MAC addresses of

devices attached to each port

Each switchport is a collision

domain

More collision domains BUT

smaller collision domains

Broadcasts still sent out ofevery port

Each switchport has dedicated

bandwidth

100% bandwidth available

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Transmission Time & Latency Bit time time taken to recognise 1 bit

Minimum frame size - 64 bytes 512 bits

Maximum frame size 1518 bytes 12,144 bits Transmission time is always 512 bit times

10Mbps 64 byte frame - 51,200 ns (100ns bit time)

100Mbps 64 byte frame 5,120 ns (10 ns bit time)

1000Mbps 64 byte frame 512 ns (1ns bit time)

Times above do not include

Time taken to propagate signal along medium

Delays introduced by hubs/switches/routers/NICs etc

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Types of TransmissionHalf-duplex

Host checks mediumfor signal if clear

host transmits Only 1 host can

transmit at a time

Collisions jamsignal generated,

back-off algorithmbefore retransmission

50-60% bandwidthavailable

Full duplex

Host can transmit

immediately 2 hosts can transmit

simultaneously

No collisions

100% bandwidthavailable

Requires dedicatedconnection to aswitchport

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Switching Concepts

Introduction to LAN Switching

Switch Operation

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Overview

Maximum availability for the least cost

Reduce the effects of collisions on available

bandwidth Reduce the effect of broadcasts on available

bandwidth

Deploy network hardware (media/switches/routers) to

overcome bottlenecks & meet bandwidthrequirements

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LAN Segmentation - bridges A bridge splits a LAN into

2 segments

It creates 2 collisiondomains

Adds 10-30% latency

Learns MAC addresses

Keeps local traffic local Forwards broadcasts

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LAN Segmentation - Switches Each switchport is a

collision domainmicro-segmentation

100% bandwidthavailable to eachswitchport

Every switchport can

send/receivesimultaneously

Host to switch/switch toswitch connection createsfull duplex link


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LAN Switch Operation - 1

When a switch starts up it sends a broadcast out

of all ports to learn host MAC addresses

When a frame is received for an unknowndestination a broadcast is sent to discover

Addresses are added to a switching table

mapping them to the port on which they werelearned

When a frame is received for a known

destination it is switched to the appropriate port


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LAN Switch Operation - 2

Switches contain RAM known as CAM

Content Addressable Memory

Stores MAC address table

Used as frame buffer

Used to queue frames in asymmetric switching

switchports operating at different speeds e.g. 10and 100 Mbps


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Switching MethodsCut-through Switching

Fast-forward as soon as destination address is readswitching starts

Fragment-free after 64 bytes have been received

(minimum valid frame size) frame is switchedStore & Forward Switching

Entire frame is received before switching Increased

Latency


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Terminology

Ignoring a frame filtering

Copying a frame forwarding

Microsegmentation dividing a network into

smaller segments (using a switch)


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Broadcasts

Bridges & switches cannot block layer 2 or layer

3 broadcasts

Adding bridges or switches to a network extendsthe broadcast domain but creates additional

collision domains a 24 port switch creates 24

collision domains

Routers can inspect layer 3 packets and create

broadcast domains a router with 3 ports

creates 3 broadcast domains

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