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CCNA version 3 Module 4 Switching Concepts

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    CCNA version 3

    Module 4

    Switching Concepts

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    Summary

    LAN congestion and its effect onnetwork performance

    Advantages of LAN segmentation in a

    network Advantages and disadvantages of

    using bridges, switches, and routers

    for LAN segmentation Effects of switching, bridging, androuting on network throughput FastEthernet 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 Ethernet is known as a shared-

    medium technology all the devicesare connected to the same deliverymedia.

    Ethernet media uses a data frame

    broadcast method of transmittingand receiving data to all nodes on theshared media.

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    Performance of a shared mediaEthernet/802.3 LAN can benegatively effected by severalfactors.

    y The data frame broadcast delivery nature of

    Ethernet/802.3 LANsy CSMA/CD access methods allow only one station

    to transmit at a time.

    y Network congestion due to increased bandwidthdemands from multimedia applications such as

    video and the Internet.y Normal latency (propagation delay) of frames as

    they travel across the LAN layer 1 media and passthrough layer 1, 2 and 3 networking devices.

    y Extending the distances of the Ethernet/802.3

    LANs using Layer 1 repeaters.

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    Half-Duplex Design

    Loopback

    TxTx

    RxRx

    Collision

    Detection

    Ethernet

    Controller

    TxTx

    RxRx

    Collision

    Detection

    Ethernet

    Controller

    Transmit

    Receive

    Loopback

    Ethernet NIC Ethernet NIC

    Ethernet physical connector provides several circuits

    Most important are receive (RX), transmit (TX), andcollision detection

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    Half-Duplex Ethernet

    Design (Standard Ethernet) The most important of these circuits

    are the receive (RX), transmit (TX)and collision detection.

    The transmit (TX) circuit is active atthe transmitting station.

    The receive (RX) circuit is active atthe receiving station.

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    To the network this appears as a single one

    way bridge. Both devices are contending for the rightto use the single shared medium.

    The collision detection circuit on each

    node contends for the use of the networkwhen the two nodes attempt to transmit atthe same time.

    When a collision occurs, a host will first

    listen to see if the network is in usebefore trying to retransmit. It willresume transmitting based on the back-off algorithm.

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    Congestion and Bandwidth To relieve network congestion more

    bandwidth is needed or the availablebandwidth must be used more

    efficiently. Throwing bandwidth at the problem.

    Attacking the symptom and not

    always the problem (illness), i.e. Couldbe broadcasts, chatty protocols,applications traffic, etc.

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    Propagation Delay

    Latency is also known as propagation delay.

    Propagation delay is the time a frame orpacket of data takes to travel from thesource station or node to its finaldestination on the network.

    The greater the number of devices the

    greater the latency or propagation delay adding hosts simply increases collisions,

    increases jam signals, and throughput willdecrease

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    Ethernet Transmission

    Times Each Ethernet bit has a 100ns

    window for transmission.

    A byte is equal to eight bits.

    Therefore, one byte takes a minimumof 800ns to transmit (8 bits at 100nsper equals 800ns).

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    A 64 byte frame takes 51,200ns or

    51.2 microseconds to transmit (64bytes at 800ns equals 51,200ns,51,200ns/1000 equals 51.2microseconds).

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    Extending Shared Media

    LANs using Repeaters Signal attenuation the signal

    weakens as it travels through thenetwork from the resistance found inthe medium.

    An Ethernet repeater to extend the

    distance of a LAN is that a singlenetwork can cover a greater distanceand more users can share that same

    network. (Coverage Area)

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    Improving LANPerformance

    The performance of a network can beimproved in a shared media LAN such

    as Ethernet by using one or more ofthe following solutions:

    y Segmenting the network using Bridges,Routers, or LAN Switches

    y Move to full duplex transmitting

    y Upgrade to the Fast Ethernet Standard

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    Why Segment LANs?

    A Cisco Segment A network can be divided in smaller

    units called segments. Each segment

    uses the (CSMA/CD) protocol andmaintains traffic between users onthe segment. By using segments in anetwork less users/devices are

    sharing the same 10Mbps whencommunicating to one another withinthe segment. Each segment is

    considered its own collision domain.

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    Why Segment LANs? In a segmented Ethernet LAN data

    passed between segments is

    transmitted on the backbone of thenetwork using a bridge, switch, orrouter.

    The backbone network is its owncollision domain and uses CSMA/CD toprovide a best effort delivery service

    between segments.

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    Segmentation with Bridges

    Bridges are different than routersbecause they are Layer 2 devices,independent of Layer 3 protocols

    they pass on data frames regardlessof which Layer 3 protocol is beingused and are transparent to theother devices on the network.

    Bridges increase the latency (delay)ina network by 10-30%.

    Why?

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    Segmentation with Routers Routers operate at the network layer

    and base all of their forwarding

    decisions between segments on theLayer 3 protocol address.

    Because routers perform more

    functions than bridges they operatewith a higher rate of latency.(Higher than other internetworkingdevices.)

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    Routers: Segment broadcast domains

    Forward packets based on destinationnetwork layer addresses

    Segment collision domains

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    More collision domains,but more bandwidthfor each user

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    Segmentation with LAN

    Switches A switch segments a LAN into

    microsegments creating collision free

    domains from one larger collisiondomain, not broadcast domains.

    With switched ethernet

    implementation the availablebandwidth can reach closer to 100%.

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    LAN Switch Latency Each switch used on an Ethernet LAN

    adds latency to the network.

    However, the type of switching usedcan help overcome the built in latencyof some switches.

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    Full-Duplex Ethernet

    Overview Full duplex Ethernet allows the

    transmission of a packet and the

    reception of a different packet atthe same time.

    Requires two pairs of conductors

    and a switched connection betweeneach node

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    Simultaneous transmission and

    reception of frames is calledbidirectional traffic (both directions)and yields 20Mbps of throughput.

    The network interface cards (NICs)on both ends need to have full duplexcapabilities.

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    Full-Duplex Ethernet Design

    Transmit circuit connects directly to receive circuit

    No collisions Significant performance improvement

    Eliminates contention on Ethernet point-to-point link

    Uses a single port for each full-duplexconnection

    TX

    Full

    Duplex

    Ethernet

    Controller

    Loopback

    TxTx

    RxRx

    Collision

    DetectionRX

    Full

    Duplex

    Ethernet

    Controller

    Loopback

    TxTx

    RxRx

    Collision

    Detection

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    Using Full Duplex

    Node must

    Be directly attached to a dedicatedswitched port

    Have installed network interface card thatsupportsfull duplex

    Full Duplex

    Half Duplex

    HUB

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    Full-Duplex Ethernet

    Design Standard Ethernet normally can onlyuse 50-60% of the 10Mbps availablebandwidth.

    This is due to collisions and latency.

    Full duplex Ethernet offers 100% ofthe bandwidth in both directions.

    This produces a potential 20Mbpsthroughput 10Mbps TX and 10MbpsRX.

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    This virtual network circuit exists

    only when two nodes need tocommunicate.

    This is why it is called a virtual circuit

    it exists only when needed and isestablished within the switch.

    Allows multiple users to communicatein parallel via these virtual circuits.

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    Source MACaddress is used

    to build thistable

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    How a LAN Switch Learns

    Addresses This means that as new addresses are

    read they are learned and stored in

    Content Address Memory (CAM). Each time an address is stored it is

    time stamped.

    This allows addresses to be storedfor a set period of time.

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    But more domains

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    Benefits of Switching A LAN switch allows many users to

    communicate in parallel through the

    use of virtual circuits and dedicatednetwork segments in a collision freeenvironment.

    Cost effective.

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    Symmetric Switching A symmetric switch is optimized

    through even distribution of network

    traffic across the entire network .

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    before forwarding

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    Asymmetric Switching Asymmetric switching is optimized

    for client-server network traffic

    flows where multiple clients aresimultaneously communicating with aserver, requiring more bandwidth

    dedicated to the switch port that theserver is connected to in order toprevent a bottleneck at that port.

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    Memory Buffering The area of memory where the switch

    stores the destination and

    transmission data is called thememory buffer.

    This memory buffer can make use of

    two methods for forwarding packets port based memory buffering orshared memory buffering.

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    Port based memory buffering

    packets are stored in queues that arelinked to specific incoming ports.

    Problem: One port may fill while anotheris empty.

    Shared memory buffering depositsall packets into a common memorybuffer that is shared by all the ports

    on the switch. (Better!)

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    3 frame transmission modes

    in a switch (+ 1 variation)

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    Three Switching Methods

    Store and Forward - the entireframe is received before anyforwarding takes place.

    Latency occurs while the frame isbeing received; the latency is greaterwith larger frames because the entireframe takes longer to read.

    Error detection is high because ofthe time available to the switch tocheck for errors while waiting for the

    entire frame to be received.

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    Cut-through the switch reads the

    destination address before receivingthe entire frame.

    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

    Switch reads the 1st 64bytes of the incomingframe beforeforwarding it to the

    destination port

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    Means theswitch is in cutthrough mode

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    Adaptive Cut Through

    Combines cut through and store andforward

    The switch uses cut-through untilthere are a given number of errors

    Then the switch will change to store

    and forward method

    Emerging Trends:

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    Emerging Trends:The Network Evolution

    Shared to Switched

    The

    New

    Wiring

    Closet

    VLAN

    System

    LAN

    Campus

    Switch

    The New Backbone

    The

    Old

    Wiring

    Closet

    HUB

    HUB

    HUB

    HUB

    HUB

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    Benefits of Switching

    Number of collisions reduced

    Simultaneous, multiple communications

    High-speed uplinks

    Improved network response

    Increased user productivity

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    Switching? Routing?

    Whats the difference? In a switching network, you'll find the

    intermediate devices keeping track of - or

    remembering - qualities of the connection. In a pure routing network, the

    intermediate devices will be indifferent toanything but handing off packets to the

    next device, and they will not be distractedby any other information, upstream ordownstream.

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    Module 4

    Switching Concepts