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Switch Question

Discussion in 'Networking' started by MichaelKing, Apr 1, 2010.

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  1. MichaelKing

    MichaelKing Thread Starter

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    Seeing how switches themselves do not have MAC addresses, when a computer on one switch wants to communicate with a computer that resides on another switch, wouldn't the 1st switch forward that frame to all nodes on it's own switch and then subsequently forward the frame to the switch with the computer with the frames corresponding MAC address? Seems to me that a switch would work like a hub in this type of scenario. Or does each switch have some sort of built in forwarding table?
     
  2. zx10guy

    zx10guy Trusted Advisor Spam Fighter

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    In general switches (unmanaged/dumb) don't have MAC addresses unless they are managed or a layer 3 switch.

    Yes, if the switch receives a frame with a destination MAC address it does not know about, it will forward that frame across all its ports. The chances of this happening is minimized as many computers/servers/devices will at least generate some traffic when it is first plugged into the network. When the particular computer/server/device generates traffic the switch will learn about the MAC address on that port. As you've said, once the switch has the MAC address in its MAC table for which port that MAC address resides on, all traffic will be directed to that port for that destination MAC address.

    No, the switch is just broadcasting for this specific scenario. There are also ARP broadcasts which get generated by clients when there is a who is ARP request. Even when there is a broadcast like this, the reply message from a device will not be broadcasted across all the ports. The other thing that makes hubs hubs is how they function electrically. They act as a splitter. So the implication of this is that the devices connected to the hub HAVE to run in half duplex. Half duplex operation allows CSMA/CD to run which is the spec in ethernet to allow for collision detection. The way ethernet operates is that a device will listen on the wire before sending a frame. If there is traffic already on the wire, the device will wait a random amount of time before checking to see if it can transmit. If the device inadvertently sends a frame when another device is already sending one, then a collision results and both devices will have to re-transmit after a random amount of time has passed. CSMA/CD handles all of this. As you can probably see, this creates performance issues due to all the wait states and re-transmits that can occur. The problem gets exacerbated as you add more and more devices to the hub or having a hub with lots of ports. There is a concept called collision domains which are typically defined by a hub or a grouping of hubs which are tied together. With switches, the collision domain gets moved to be just between the switch and the connected device. Because the likelihood of a collision occurring in this scenario is slim to none, CSMA/CD does not need to run and the device can talk to the switch at full duplex.
     
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