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Frame Flooding ambiguity

Discussion in 'Routing & Switching' started by Raider, Nov 19, 2007.

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  1. Raider

    Raider Bit Poster

    Hi everyone

    I have a scenario which is confusing me, i believe the answer is what i am guessing but am not sure. So here goes....

    A switch doesnt know of any of the MAC addresses of the PCs connected to it or in other words the switching table is empty. There are six computers connected to the switch at the moment . When the switch recieves the fist frame from a computer 1 it adds the source MAC and floods the frame over all the interfaces except the source interface. When the other i.e. the destination machine (2) replies with a frame the switch adds its MAC address alongwith its interface to the switching table as well.Now, the switching table has 2 entries for machine 1 and 2 but still no entry for machines 3,4,5,6. Now if machine 3 sends a frame to machine 4 and the switch adds the MAC for machine 3 in its table, will the frame be flooded to machines 4,5,6 as the switch knows about machines 1,2 and 3 is the source machine.

    I hope i made sense... i know the answer must be simple but i want to be sure.

    cheers guys....
  2. Liqua

    Liqua Bit Poster

    Hello Raider,

    Everytime a switch receives a frame destined for somewhere it does not know about it is flooded to all other ports (except the one it was received on) regardless of what's in the MAC address table.

    In your example then computer 3 sends the first packet to the switch destined to computer 4, as the switch does not know where computer 4 is (and it could be on a hub sharing port 1 for all it knows) the frame is flooded to all ports except the one on which computer 3 was. (And it adds in computer 3's MAC address at the same time !)

    Hope that helps.

    Certifications: CCNA, SND, ITIL Foundation
  3. Spice_Weasel

    Spice_Weasel Kilobyte Poster

    Close, but not quite - the frame from machine 3 to machine 4 will be flooded out every interface, except the interface the frame was received on. The fact that the switch knows computers 1 and 2 from previous frames sent does not matter - after all, as far as the switch is concerned any interface could be connected to multiple hosts, for example another switch or hub with multiple machines. Therefore, the switch must flood the frame out all ports (except the one that the frame arrived on) to ensure it can reach its destination.


    *edit* Liqua beat me to it ;)
    Certifications: CCNA, CCNP, CCIP, JNCIA-ER, JNCIS-ER,MCP
  4. Raider

    Raider Bit Poster

    thanks both of you guys,,,,you made me a happy boy...:D

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