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I think this is a pretty simple question, but I wanted to either validate my thinking, or for someone to tell me I'm wrong.

My router at home has 4 gigabit ports. So, obviously, each (supported) device connected to it is over a gigabit connection.

If I add a gigabit switch to one of these ports, and I add two devices to the switch, will they still have gigabit throughput to the rest of the LAN, or will it be reduced because there are two devices fighting for contention over the gigabit connection from the switch to the router?

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Let's say the two devices are named dev1 & dev2, the links from them to the switch respectively L1 & L2 and the link from the switch to the router L3.

The two devices will have to share the gigabit, meaning that if only dev1 is transmitting, dev1 will have 1 gigabit for itself but if the two of them are transmitting at the same time, they will most probably have 500 Megabit each.

If we speak about capacity of each link, capa(L1)=capa(L2)=capa(L3)=1 Gigabit (if your wires are indeed gigabit compliant). So the maximum throughput you can have on l3 is 1 gigabit.

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Keeping in mind that most consumer marketed switches do not have a sufficiently capacious backplane or buffers to actually provide simultaneous Gigabit to all its ports, yes, downstream bandwidth will be choked by constraints imposed by upstream switching. at the same time, many hosts can't handle streams at full gigabit speeds for most tasks anyway.

One way to avoid this is to aggregate several ports between the switches so you can get 2x or more bandwidth between the switches. https://en.wikipedia.org/wiki/Link_aggregation you will need a managed or Smart switch to set up LAG.

  • So how does this work when you have a server and (say) 20 PCs on a network switch... the server-switch link is surely going to totally strangle your lovely Gigabit if people are actually using the network? Doesn't this negate the benefit of the switch if ideally all the devices would just plug directly into the server? – Mr. Boy Mar 6 '18 at 14:58
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    thats why link aggregation (or Teaming) is valuable, in that it allows you to multiply available bandwidth to the server from the server switch, so a desktop has 1Gbps, and the server has 4 or 10Gbps. That said, keep in mind that the servers network card can only process one command at a time, so there is always an element of time-slicing in contentious environments, whether you have a bandwidth issue or not. – Frank Thomas Mar 8 '18 at 13:45

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