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If I have a 24-port gigabit switch with no uplink, and the odd-numbered downlink clients are transferring data to the even-numbered downlink clients, will I theoretically have (12 x 1 gigabit) communication channels, or (1 gigabit / 12) communication between each pair?

What features would a switch need to have to achieve the 12x1 performance if it doesn't already do so?


Update: From this closed ServerFault question I've learned that to calculate the required backplane capacity I would need to calculate 24*1 Gbs or 24 GBs capacity. For theoretical maximum, it would be 24*2 Gbs = 48 Gbs. If I'm searching for a switch, what marketing terms should I look for to avoid purchasing an under-capacity switch?


Update: The positive marketing terms I've found are "non-blocking", "no output contention", "100% throughput", and "wire speed" (plus checking the backplane capacity).

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Generally, modern switches are "non-blocking", ie. they can forward any bandwidth not exceeding the capacity of the output port(s). (With the possible exception of large, chassis switches with very high-speed ports.)

In your example, you should be able to run the full port speed across any pairs you want simultaneously - 12x 1 Gbit/s in duplex. Check the switch specifications for details. This is usually listed as "48 Gbit/s" (counting each ingress and egress bit and running in each direction).

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  • What marketing terms should I look for to find such a switch and avoid purchasing one with a lower backplane capacity? Any terms to avoid? e.g. "fast ethernet" = "very slow ethernet" today – Drakes Aug 21 '18 at 16:59
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    "Fast Ethernet" is the second generation with 100 Mbit/s - rather slow today, but still the basic standard. "Non-blocking" or "wire-speed" is what you want. And make sure it's got the right number of Gbit ports. – Zac67 Aug 21 '18 at 17:02
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The answer will be different for every model of switch. Some switches are designed to process every port at wire speed, but others will oversubscribe the backplane to save money.

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  • I didn’t know that. What terminology should I look for in the switch that does the 12x1? – Drakes Aug 21 '18 at 16:24
  • You might see the term "non-blocking." Also, the backplane speed has to be higher that the sum of all the port speeds. – Ron Trunk Aug 21 '18 at 18:03

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