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This one might be the dumber question ever asked in this place, but here it is:

If I have 20 machines, all equipped with gigabit ethernet NICs, all connected to a gigabit switch, is the maximum volume of data going through the switch per second equal to 1Gbps, or is it more like 1Gbps * 20?

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If I have 20 machines, all equipped with gigabit ethernet NICs, all connected to a gigabit switch, is the maximum volume of data going through the switch per second equal to 1Gbps, or is it more like 1Gbps * 20?

It is closer to 1Gbps * 20.

Throughput is usually written in the documentation and is expressed in packets per second, which can be translated to mega/gigabits per second depending on packet sizes.

It really depends on the switch, its intended use, price, etc.

For example, cheap SOHO 8 port L2 gigabit switches usually get throughputs from 1–5Gbps (on all ports together — out of 16Gbit full-duplex theoretical max). Enterprise/datacenter/ISP switches can do a lot more.

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  • Thx mulaz; I'll ask the osting provider about it. – arnaud briche Oct 15 '13 at 9:39
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    @Mulaz, +1 but just a small quibble with your answer... you said throughput is expressed in packets per second and translated to Gbps... the reality is that switches have limits based on both PPS capacity (from their central forwarding engine) and Gbps (from their switching fabric). See "What is the exact meaning of each figure? What is the difference between them?" for details – Mike Pennington Oct 15 '13 at 10:20
  • Yes that is true, but I have mostly(!) seen PPSs in switch datasheets, and especially with cheaper switches, the PPS limitation can bring the whole bandwidth even udner 1Gbps (small packets, voice, some p2p services, rt gaming) – mulaz Oct 15 '13 at 11:15
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    Wouldn't this actually be 2Gbps * 20? with full-duplex if the switch has the backplane b/w needed, plus all the other important stuff (buffers, asics etc).. – pauska Oct 15 '13 at 18:43
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    Mike Pennington is right about frame rates, but not right to say it's only a small quibble. Because of the Ethernet preamble (8 bytes per frame) and the gap between frames (typically 12 bytes), the bandwidth available for user traffic can be much less than 1 Gbit/s per port. With 64-byte frames at line rate (1.488 mpps), the actual bandwidth used is only 761.9 Mbit/s, not 1 Gbit/s. Even with maximum-length 1518-byte frames, the bandwidth utilization is 987 Mbit/s, not 1 Gbit/s. Yes, the layer-1 rate really is 1 Gbit/s, but it's not all available to user traffic. – user8162 Oct 15 '13 at 23:31
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To really be specific here, a full line rate switch [please refer to specific switch documentation to determine what your specific fabric can handle] with 20 one gigabit connected machines on it will have a max throughput of...

20 machines * (1 gigabit * 2) = 40gbps

This is due to traffic being full duplex.

A single one gigabit device can account for two gigabit on the switch. One gigabit going into the switchport from the host and one gigabit leaving the switchport towards the host.

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