ccna3 chapteer 4 - switching concepts

CCNA3: Switching Basics and Intermediate Routing v3.0

CISCO NETWORKING ACADEMY PROGRAM

Switching Concepts

Introduction to Ethernet/802.3 LANsIntroduction to LAN Switching

Switch Operation



Switching Concepts

Introduction to Ethernet/802.3 LANs



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




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!




Hubs• Layer 1 devices• Regenerate, retime,

amplify signals• 1 collision/bandwidth

domain• Broadcasts propagated

out of every port• Only 1 device can

transmit at a time• Only 50-60% bandwidth

available




Bridges• Layer 2 device• Splits network into 2

collision/bandwidth domains

• Broadcasts are forwarded• Local traffic stays local• Checks Layer 2 MAC

addresses in 802.3 frame




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 of

every port• Each switchport has dedicated

bandwidth• 100% bandwidth available




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




Types of TransmissionHalf-duplex• Host checks medium

for signal – if clear host transmits

• Only 1 host can transmit at a time

• Collisions – jam signal generated, back-off algorithm before retransmission

• 50-60% bandwidth available

Full duplex• Host can transmit

immediately• 2 hosts can transmit

simultaneously• No collisions• 100% bandwidth

available• Requires dedicated

connection to a switchport



Switching Concepts

Introduction to LAN Switching Switch Operation




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 bandwidth requirements




LAN Segmentation - bridges• A bridge splits a LAN into

2 segments• It creates 2 collision

domains• Adds 10-30% latency• Learns MAC addresses• Keeps local traffic local• Forwards broadcasts




LAN Segmentation - Switches• Each switchport is a

collision domain – “micro-segmentation”

• 100% bandwidth available to each switchport

• Every switchport can send/receive simultaneously

• Host to switch/switch to switch connection creates full duplex link




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 unknown

destination a broadcast is sent to discover• Addresses are added to a switching table

mapping them to the port on which they were learned

• When a frame is received for a known destination it is switched to the appropriate port




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. 10 and 100 Mbps




Switching MethodsCut-through Switching• Fast-forward – as soon as destination address is read

switching starts• Fragment-free – after 64 bytes have been received

(minimum valid frame size) frame is switchedStore & Forward Switching• Entire frame is received before switching In

crea

sed

Late

ncy




Terminology• Ignoring a frame – filtering• Copying a frame – forwarding• Microsegmentation – dividing a network into

smaller segments (using a switch)




Broadcasts• Bridges & switches cannot block layer 2 or layer

3 broadcasts• Adding bridges or switches to a network extends

the 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

ccna3 chapteer 4 - switching concepts

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