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I'm running a simulation in Riverbed modeler, and I've got two broadcast domains, each having one Ethernet advanced hub and 16 Ethernet advanced stations connected to it through 10Base-T links. Subsequently, another 10Base-T link has been adopted to connect together the hubs and switch, whereby the switch (ethernet advanced switch) would be in the middle of both hubs.

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After executing the simulation the results are like this;

  • Traffic forwarded by switch : 4 Mbps
  • Traffic received by switch : 8 Mbps
  • Point to point through switch to hub1 : 4 Mbps
  • Point to point through switch to hub2 : 2 Mbps

I've studied all the possible factors, such as collision count in collision domains, queuing delay for point to points, and the questions are:

  • Why does the switch behave in this manner?
  • Why does the traffic being forwarded to hub1 vary from the traffic being forwarded to hub2?
  • Why is the traffic on the point-to-point throughput to hub2 2 Mbps, whereas the traffic on the point-to-point throughput to hub1 is 4 Mbps?

I've read all about the switch technologies, but I haven't found any relative topic that describes how a switch behaves in such cases. Any source or suggestion would be highly appreciated.

This scenario is in an office (100m x 100m).

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    You don't have separate broadcast domains; you have two collision domains (2 hubs) Also, unless you are working in a 30 year old network, there won't be any hubs. – Ricky Beam Apr 5 '16 at 3:28
  • Yea, I'm aware that hub is archaic, but isn't that also a broadcast domain? hmm, if no, what makes it not a broadcast domain ? – Pouya Ataei Apr 5 '16 at 3:37
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    Any broadcast frame anywhere in that network will go everywhere. A switch does not stop broadcasts, but repeats them ("flood") to every other port. (until vlans enter the equation, which does create different broadcast domains) – Ricky Beam Apr 5 '16 at 3:44
  • Put another way, a switch handles broadcast in the same manner a hub handles every frame. – Ricky Beam Apr 5 '16 at 3:45
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You don't actually have any point-to-point links. A hub is a layer-1 device, basically, a multi-port cable, while switches are layer02 devices which only deliver frames to where they are destined. All your links are shared links.

Unlike hubs, switches don't forward every frame out every port. Switches learn which MAC addresses are on which ports, and they deliver traffic destined for a particular MAC address to the port where that MAC address is found. Also, unlike hubs, each switch port is a collision domain, and there will be no collisions on the switch, itself.

You must also be sure that the switch ports are not configured for full duplex. Otherwise, you will get a lot of errors. Ethernet has no facility for retransmission for damaged frames; they are just dropped.

  • Thank you very much for the explanation, I still don't understand the relationship between the traffic forwarded and received from the switch with the throughput of links connected to the hub ? Our switch detects hubs mac addresses, and deliver traffics, but why in this imbalanced manner? why double the amount of hub2 is forwarded to hub1 ? – Pouya Ataei Apr 5 '16 at 2:36
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    Hubs don't have MAC addresses, they are like cables. Without more detail about what is being sent to where, you can't really answer that with any certainty. Also, you said this is a simulation, so it is only as good as the programmers made it. It could be that whichever side started first has a big advantage. If this was real equipment, I would suggest you look at the switch port statistics and counters. – Ron Maupin Apr 5 '16 at 2:41

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