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When network switches report speeds of say 100 Mbit or 1 Gbit, are they referring to the maximum speed per Ethernet port or is this the physical limit of the switch on all ports? Say for example two users on a network are simultaneously transferring large files, will they be contending against each other for the 100 Mbit or 1 Gbit, or will they each attain these maximum transfer rates?

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up vote 8 down vote accepted
  • Each port has a maximum speed of e.g. 1 GB/sb in one direction, so sending to two other machines will limit the overall send throughput to this value. It could receive the same amount from other machines because of the full-duplex transfer.
  • Switches have a limit of how much traffic the backplane can handle. It is usually lower than the theoretical combined bandwidth (which is the speed of a single port x 2 [full-duplex] x the number of ports of the switch). This backplane limit is usually listed in the specs of the switch.
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In one of the data sheets I came across "Switching Capacity: 20 Gbit/s". Is this likely to be the backplane limit? The switch in question has 10 ports @ 1 Gbit/s – QFDev Jul 7 '13 at 11:02
Yes, that's it. – Sven Jul 7 '13 at 11:03
@QF_Developer Clearly, if a switch has 10 ports at 1Gb/s, the maximum possible total switching capacity is 20 Gb/s, and that assumes you count traffic twice, coming in and going out. I would interpret a 10-port gigabit switch that claimed a 20 Gb/s switching capacity as claiming to be fully non-blocking, that is, the backplane will not be the limiting factor. – David Schwartz Jul 7 '13 at 11:35

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