Hub versus switch

Discussion in 'Windows Networking' started by Spin, Feb 3, 2006.

  1. Spin

    Spin Guest

    Experts,

    Given a hub with five computers connected to it. One computer broadcasts
    and all others hear it. Now, given a switch with five computers connected
    to it. One computer broadcasts. Do all others hear it? Why or why not?
     
    Spin, Feb 3, 2006
    #1
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  2. In
    A switch is an OSI Layer 2 device (based on the MAC address). The higher you
    go up the OSI model, the more intelligent the device is that works on that
    layer. For example, a router is a Layer 3 device (the Network Layer where IP
    lives).

    Therefore, being intelligent enough to be aware of the MAC addresses, a
    switch builds a "routing" table based on MAC addresses it is aware of when a
    device is plugged into a port, but not IPs, since it's on the wrong layer.
    HOwever at times, I don't usually like to use the word 'routing' in this
    definition because it can be confused with a router, so we should look at it
    as a 'destination lookup table' that keeps track of each port and the MAC
    sitting on it. Some switches, like the Ciscos, can build a table with about
    40,000 entries, if I remember correctly, and has a propeitary shared memory
    pool that is able to transfer data independently between ports more
    efficiently than other brands. Others don't do as well. Yes, you do get what
    you pay for. Anyway, a switch will look at the source MAC and the
    destination MAC in the packet. If the destination is a broadcast, make it
    so. If it's directed transmission, say from a machine with a MAC address of
    MAC1 on port 32 to a machine with a MAC address of MAC5 on port 15, then the
    trafffic directed from only port 32 to port 15 and no others will 'hear' or
    'see' that traffic.

    Hubs are just repeaters that repeats traffic on all ports and have no
    intelligence... cheapos now days.

    btw- There are Layer 3 switches too. They are switches that you can define
    each port to be 'switched' or routed, such as an IP router. They are of
    course more expensive. I believe the Catalyst 2850 are Layer 3 switches,
    where we can create IP subnet VLANs.

    The higher you go up the OSI, the more intelligent, and the more expensive
    the device is.

    Hope that helps.

    --
    Ace

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    Ace Fekay, MCSE 2003 & 2000, MCSA 2003 & 2000, MCSE+I, MCT, MVP
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    Ace Fekay [MVP], Feb 3, 2006
    #2
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  3. Spin

    alexk Guest

    Yes.

    The Ethernet standard states device share the eletrical bus between them. A
    tool known as CSMA/CD is an arbitrator determining who has access to the
    network at the moment. Since only one device can transmit at a time, each
    machine must wait in turn before they can send a packet. If two devices were
    to send a packet and an eletrical collision was to occur, the sending devices
    would sense the collision and retransmit their data. A hub can be seen as a
    tool that shares the bus. It allows multiple devices to operate on the same
    bus within the limits of CSMA/CD.

    A switch extends the power of a network by electrically isolating each port
    on the switch thereby eliminating the need for CSMA/CD in a sense (still
    necessary if you hosts attached to a hub which is attached to a switch). The
    only two devices operating on the wire are the sender and the receiver. This
    allows you to configure data transmission to occur in a full duplex manner
    allowing you to achieve double the original throughput. Plus you no longer
    have to wait your turn. The drawback is that the switch must analyze the
    packet- slowing down the overall speed of a network. Today though most
    switches operate more efficiently then hubs with more than four attached
    devices.


    Layer 3 (routers and layer 3 switches- VLANs) devices will help to elimate
    broadcasts.
    Hope this helps.

    alex k
    alex k
     
    alexk, Feb 3, 2006
    #3
  4. Yes.
    Switches are Layer2,..they stop "collisions" not broadcasts.
    Broadcasts are stopped by Layer3 devices (routers).

    --
    Phillip Windell [MCP, MVP, CCNA]
    www.wandtv.com
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    Phillip Windell, Feb 3, 2006
    #4
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