Practice problem about a switch and it's database

By Jskid ยท 6 replies
Oct 17, 2012
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  1. I'm taking a networking class and am stuck on a practice problem and was hoping someone could give me some guidance.


    The first time port 1 receives information from port b. The second time port 2 receives information from source e. The third time port 1 now receives information from part c (over writing the data from the first time). How do I know ports 3 and 4?
    For the second part of the question, the switch would transmit data on just the port connected to e, right?
  2. jobeard

    jobeard TS Ambassador Posts: 11,168   +986

    Boy that is interesting, but imo not very practical. Please don't get me wrong, you're learning good stuff
    and you will certainly benefit in the long run. But in truth, many classes are taught (in all fields of education)
    because they can, not because the information is useful. I took a curriculum in which there were many classes
    which were there only to round out the number of units to qualify for the degree.

    Surely you've wondered "how can I use this?". A class on Router Design would be more beneficial.

    Best wishes, Jeff
  3. Jskid

    Jskid TS Guru Topic Starter Posts: 346

    I've figured it out with one last question.

    What I don't get is this: (btw this online "quiz" told me if I'm right or wrong)
    The column for a port can have multiple forwarding mac addresses.
    e.g. port 1 receives a packet from mac A and then receives a packet from port B. mac A and B are now in the column for port 1 (this is ridiculous because obviously there can be only one device connected to port 1)

    In contrast, following from the above example, if mac B had previously been in the column for devices attached to port 2, it would now be deleted from that row (which makes sense).

    Is this how it really works and why are there multiple forwarding addresses for one port, why not drop the old ones?
  4. jobeard

    jobeard TS Ambassador Posts: 11,168   +986

    The clue is the DESTINATION MAC, not the source port :)
  5. jobeard

    jobeard TS Ambassador Posts: 11,168   +986

    Btw: forwarding to ONLY the destination MAC is the fundamental difference 'twix a hub & switch
  6. Jskid

    Jskid TS Guru Topic Starter Posts: 346

    I am wondering something. If a switch doesn't know what port to transmit on why does it transmit on all ports accept for the one the packet came from, shouldn't it only be ports where the destination is unknown? For example if a package has destination address A and it came from port 1 and you know B is on port 2 and C is on port 3, through process of elimination wouldn't you know to send the packet to port 4?
  7. jobeard

    jobeard TS Ambassador Posts: 11,168   +986

    I don't believe that to be true - - that's the behavior of a HUB
    That might be useful.

    A fundamental difference between hubs and switches is the latter provides the down-link side of the connections far less 'noise' and useless traffic (ie: why send packets for A to anyone else).

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