CCNA3_M4_Switching_Concepts (1130761)

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CCNA – Semester3Module 4Switching ConceptsObjectives• Introduction to Ethernet/802.3 LAN• Introduction to LAN switching• Switch operationIntroduction to Ethernet/802.3 LANCollisions in Ethernet• Collisions are a by-product of Ethernetnetworks.• This indicates that the network has become toocongested or too many users are trying toaccess the network at the same time.Layer 2 SwitchesLayer 2 and layer 3• The disadvantage of Layer 2 devices is that theyforward broadcast frames to all connecteddevices on the network.• When the number of broadcasts in a network isexcessive, sluggish network response timesresult.• The router makes decisions based on groups ofnetwork addresses, or classes, as opposed toindividual Layer 2 MAC addresses.Routers• The purpose of a router is to do all of thefollowing:– Examine incoming packets of Layer 3 data– Choose the best path for them through the network– Switch them to the proper outgoing port• Routers are not compelled to forwardbroadcasts.• Routers are the most important traffic regulatingdevices on large networks.Factors that impact network performance• The multitasking environment present in currentdesktop operating systems allows forsimultaneous network transactions.• Network intensive applications such as theWorld Wide Web is increasing.• Client/server applicationsEthernet/802.3 InterfaceCSMA/CD AlgorithmHost wantsto transmitBroadcastjam signalIs carriersensed?AssembleframeStarttransmittingIsa collisiondetected?attempts =attempts + 1Attempts> toomany?Wait fort secondsAlgorithmcalculatebackoffKeeptransmittingIstransmissiondone?Too manycollisions; aborttransmissionTransmissioncompletedHalf-Duplex Ethernet• Legacy Ethernet is a half-duplextechnology.• As more hosts are added to the network andbegin transmitting, collisions are more likelyto occur.Network-intensive applications• Today's networks are experiencing an increasein the transmission of many forms of media:––––Large graphics filesImagesFull-motion videoMultimedia applications• Results in slower response time, longer filetransfers, and, hence, less productive.• Solutions:– More bandwidth is needed OR– Available bandwidth must be used more efficiently.Congestion and bandwidthNetwork latency• Latency, sometimes called delay.• It is important to quantify the totaltransmission latency.• Latency has at least three sources:– NIC delay (around 1ms for 10BASE-T NICs).– Propagation delay.– Latency in networking devices.• Latency does not depend solely ondistance and number of devices.Ethernet 10Base-T transmission timeWHY ?Full-duplex Ethernet• Full-duplex mode allows two stations tosimultaneously exchange data over a pointto point link• Provides independent transmit and receivepaths.• Bypasses the CSMA/CD protocol.• Full-duplex Ethernet offers 100% of thebandwidth in both directions.• Results in doubled aggregate throughput ofthe link.Full-duplex media• The physical medium must be capable ofsupporting simultaneous transmission andreception without interference.– Supported: 10Base-T, 100Base-TX, 100Base-FX,and 1000Base-T.– Not supported: 10Base5, 10Base2, and 100Base-T4.• End stations’ NICs must be capable of fullduplex operation.• Full-duplex operation is restricted to point topoint links.Fast Ethernet 100Mbps• Transmission speed is 100Mbps.• Three main media varieties:– 100Base-TX: Two pairs of twisted-pair cable– 100Base-T4: Four pairs of twisted-pair cable– 100Base-FX: Fiber optic• Introduces Auto-Negotiation mechanism ofthe media speed.Introduction to LAN switchingLAN SegmentationSegmentation with bridges• Bridges increase the latency (delay) in a network by 1030%.• A bridge is considered a store and forward device.Segmentation with switches•••Results in faster transmission and more efficient use of bandwidth.Micro-segmentation, can reach 100% bandwidth utilisation.Low latency levels and a fast frame forwarding speed (switching isimplemented in hardware )Segmentation with routers• Routers operate at the network layer.– Routers makes packet forwarding decisions based on the Layer 3protocol address.• Layer 3 operations adds more latency.• Adding a latency factor of 20% to 30% over a switchednetworkLayer 2 Switching• Layer 2 switching is based on MAC addressinformation.Layer 3 Routing• Layer 3 routing is a function of the networklayer.• Layer 3 routing: layer 3 header information isexamined and packets are routed afterconsulting routing table.• The process is software-basedLayer 3 switching• Layer 3 switching:layer 3 headerinformation isexamined and packetsare switched usingspecialized hardwareASICs for high speedand low latency.Symmetric switching?Asymmetric switchingMemory buffering• Is an Ethernet technique to store packets tobe forwarded to the correct ports.• Buffer is an area of memory where theswitch stores incoming frames.• Effectively used when the destination port isbusy.• Two methods for forwarding packets:– port-based memory buffering– shared memory bufferingTwo switching methods• Store-and-forward:– The entire frame is received before forward.– Increases latency with larger frames.– Error detection is high.• Cut-through:– Fast-forward switching:• Immediately forward a packet after checking the destinationaddress.• Offers the lowest level of latency– Fragment-free switching:• Forwards a packet after receiving the first 64 bytes.• Filters out collision packets (size < 64 bytes)ForwardingSwitch operationLAN Switches• Replace shared hubs and are designed towork with existing cable infrastructures.• Perform two basic operations:– Switching data frames: Frame arrives on an inputand is transmitted to an output media.– Maintaining switching operations: Switches buildand maintain switching tables.Content addressable memory (CAM)••••Records MAC address of the source station with a timerstamped with itSends out frame to all ports (except frame’s input port) whendestination is broadcast, multicast or unknownForwards when dest.

is on different segmentFilters if dest. is on the same segmentFrame transmission modes• Another transmission mode (hybrid mode) is calledadaptive cut-through, the switch uses cut-throughuntil it detects a given number of errors. Once theerror threshold is reached, the switch changes tostore-and-forward mode.Intersubnet••Challenge: intersubnet communicationSolution: use routers or add Layer 3 capability to switchSwitches and workstationSummary•••••••Collisions in an Ethernet networkElements affecting network performanceThe basic operations of a switchSymmetric and asymmetric switchingMajor switch frame transmission modesThe process by which switches learn addressesMicrosegmentation using switchingCCNA3 – Module4.

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