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Edit: it seems I either misunderstood or was misled by this article, which seems to imply that a 10 Gbps switch is required to make good use of a dozen nodes with gigabit ports. With the clarification that 10/100/1000 generally refers to link speed, the rest of the question doesn't make much sense anymore!


I am new to networking, and I am having trouble finding resources on how traffic is routed through an ethernet network. As I understand it, most gigabit-compatible devices can handle the full gigabit bandwidth through a single interface, and even cat6 cables can handle 10 Gbps--so neither interfaces nor links are likely to be the rate-limiting components of a network. But it is unclear to me exactly how the limits that do exist apply. I have a few diagrams to illustrate.

Three network topologies

Say I have 1 gigabit of data on node 1, and I want to send 1/3 gigabit of it to each of the other nodes.

  • Network A is simple and seemingly pretty standard. Assuming that the switch can only handle 1 Gbps of net traffic (I've read articles that seem to imply this, but I've never seen it stated explicitly), it is clear that the transfer cannot be faster than 1 second, with node 1 sending at 1 Gbps and nodes 2-4 receiving at 1/3 Gbps each.

  • In Network B (two switches), is it the case that 1) traffic is distributed through both switches so the transfer is completed twice as fast as in Network A, or 2) the network protocol as a whole can only handle 1 Gbps of data and the transfer rate is unchanged?

  • In Network C (fully-connected), where each node has many interfaces, does the communication protocol enforce a bandwidth limit, or is the interface speed the only bottleneck?

Note: I am aware that this is an oversimplified case--that in some cases, links and interfaces can be bottlenecks, and that I've ignored lots of issues with overhead, theoretical vs. real bandwidths etc. But I don't believe those are necessary for the question I'm asking.

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No (to "the switch can only handle 1 Gbps of net traffic"). Speaking very generally to your question, expect that each physical link can transfer duplex data at the stated bandwidth rate for the switch.

Why

In the context of industry, the speed metric (10M/100M/1G)bps directly refers to the link speed; i.e. under laboratory conditions, the line into this device is rated to operate at a maximum of 1Gbps. This does not imply it WILL be 1Gbps, that it cannot exceed 1Gbps, or even (usually) that it can sustain 1Gbps at all times.

As to the multiplexing or switching of the data from one internal device link to another, this is typically never a processor limitation since CPU speeds are usually an order of magnitude greater than link speeds (1Gbps = 125MBps; 1Ghz even on a hypothetical 1 byte bus is still > 125MBps; in the world of 32-bit and 64-bit buses you will never lag your processor).

Where you will see failure is in your memory queue, which is pretty obvious when you think about it. You have a finite amount of memory, but in a 4-port 1Gbps switch you have the possibility that 3 of your 1Gbps ports are receiving data and only 1 1Gbps is sending data. You clearly cannot sustain an overage of 2Gbps in your network queue for very long before you are going to run out of memory and start dropping packets.

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    This whole answer is helpful, but the first sentence clarifies a major misunderstanding that I had about the way switches worked. I added an edit to reflect that. – KPM Sep 19 at 20:49
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Link speed (1gbps) is a physical interface speed. The protocols that a network uses have no concept of speed and is independent of the physical medium, they simply use the physical infrastructure.

While some switches may be limited to 1Gbps they would cause a bottleneck when multiple devices are connected and talking independently. While multiport switches might not support the transfer of 1gbps in both directions on every port at once I would be surprised if they are limited to a total of 1gbps.

For network 1, yes, a saturate link on one port will limit the speeds of other devices trying to get data from that machine. You will effectively get (1 / (number of transfers)), or in your case 1/3 of the link speed.

For network 2, if the computers all support and are properly set up for link aggregation (combining independent links to emulate a single path) then you will have an effective link speed of 2gbps to any given machine. If they do not support or use aggregation then they may simply use whichever link they found worked first. It depends how the network is set up.

For network 3 the link speed between machines is their transfer speed.

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Assuming that the switch can only handle 1 Gbps of net traffic (I've read articles that seem to imply this, but I've never seen it stated explicitly)

That's a incorrect assumption, which I presume you added. A switch will have a dataplane speed, this is the speed it can move data between ethernet ports. For my home 1Gb switch, it has a dataplane speed of 16Gbs, and while I doubt I'd ever reach this, its enough for most practical applications. For business purposes, you'll generally want switches with higher dataplane speeds depending on your requirement.

Now, onto the 'limit' of the speeds - this is limited by the interface, it's also limited by the cable length and quality, how many pairs are connected in the cable all of which cause the interface to decide what speed it can connect at. If 4 pairs are detected in a cat5e (or better) cable, the interface will attempt to run at 1Gbps. If only 2 pairs are detected, then it'll limit to 100Mbps.

  • It seems that you're saying this article is wrong or misleading when it recommends using a 10 Gbps switch to connect a set of nodes that each have gigabit ports? If so, that is very helpful to have cleared up. – KPM Sep 19 at 20:41
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    @KPM: The article isn't wrong – it talks about the link to the outside network, which is going to be a single uplink port shared by all devices. The dataplane speed allows for 16 Gbps in the ideal case (let's say if you have ports 1↔3 communicating, ports 2↔4, ports 5↔6, and so on – each pair can speak at full 1 Gbps), but it can't help at all when multiple input ports try to share same output port (let's say you have 1↔9, 2↔9, 3↔9, and so on – they now must share port 9's limit). – grawity Sep 20 at 5:07

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