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I'd like to understand a little more on the Ethernet switches.

Consider this Ethernet Switch which is a 3 port switch.

Like, it has 2 copper ports and one digital (MAC side interface) port.

It is mentioned as Gigabit ethernet switch port. In that case, when the 2 Cu ports have 1Gbps input data coming, (so total 2Gbps of data), will the output MAC port also be able to provide the data at 2Gbps or 1Gbps?

What is the purpose of a switch in case if it can provide only 1Gbps MAC data output when 2 Cu ports have 2Gbps input data but the MAC side or the switch can only transmit data at only 1Gbps speed?

I understood about the Non-blocking term from my previous question. But in this case, if the switch can provide only 1Gbps MAC output, does it really make any sense to say the switch is non blocking?

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2 Answers 2

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It is a switch and that is how Ethernet works, it is a packet switched network, so the purpose of the switch chip is to switch packets between interfaces.

All three ports are gigabit, and if it says non-blocking, it means it can route packets between them as fast as it can long as the bandwidth fits through the ports. So obviously you can't send 1Gbps traffic from two ports simultaneously to one 1Gbps port because it would require 2Gbps which does not fit through a 1Gbps port.

And yes it makes sense to say switch chip is non-blocking because it is non-blocking if it can switch 1Gbps from port 1 to port 2 and another 1Gbps from port 2 to port 1 so two ports having full bandwidth in both directions. So it means that the switching fabric between interfaces can handle everything that fits through the 1Gbps ports.

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  • \$\begingroup\$ Thank you for the answer. Just a small follow up question: In the Same 3 port Ethernet switch that is having 2 Copper port and 1 Digital port, Can the packets go from 1 Copper port to another copper port? Or, should the packets go from the copper port to digital port only? Like, just wanted to know the directionality of the data transmission. \$\endgroup\$
    – user220456
    Commented Apr 21, 2022 at 3:47
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    \$\begingroup\$ It's a 3 port switch. What types of ports it has is irrelevant. And because it is a configurable switch, you can define how it operates. \$\endgroup\$
    – Justme
    Commented Apr 21, 2022 at 4:41
  • \$\begingroup\$ Oh, so we can communicate and exchange data packets between the Cu ports alone, also, without any use for the MAC port? I'm asking because, I saw that only the MAC port is configurable with the MII, RMII & RGMII interfaces, but not the entire switch? Am I understanding correctly? \$\endgroup\$
    – user220456
    Commented Apr 21, 2022 at 6:00
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    \$\begingroup\$ It's a switch, that is what switches do, they switch packets between ports, and it is a configurable switch, which means you can configure how it switches packets between ports, regardless of any interface used. It has three ports, two ports with built in copper PHYs and one port without a PHY to connect it to something else. \$\endgroup\$
    – Justme
    Commented Apr 21, 2022 at 6:30
  • \$\begingroup\$ Thank you for the answer! \$\endgroup\$
    – user220456
    Commented Apr 21, 2022 at 7:02
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If the two copper ports are forwarding 500Mb/s to each other AND 500Mb/s to the GMII port then all three ports are fully loaded so yea it can make sense.

If ports 1 and 2 are copper and 3 is GMII then you can reasonably do something like say plug port 1 into the wall jack to the upstream switch, plug port 2 into the back of my office PC and use port 3 to talk to the little micro inside my desk phone. The Lions share of the traffic is then port1 <-> port2 with only a little traffic to port3, but the port 1 and 2 really NEED to be 1G to avoid slowing that link down, and port3 may as well be 1G as it keeps all the switching core running at the same speed.

The common use case for little three port switches like this is in such things as IP phones, where it is useful to be able to insert the thing into an existing ethernet circuit.

Granted that doesn't need 1G on the GMII port, but in an office environment a 1Gb/s LAN is very, very common so I would hope that looping my desk phone into the cable going to the PC would not significantly slow down the connection to the PC, so those two ports do need to be 1G, even if the GMII port was actually 100M or even 10M.

Obviously it you try to route more bandwidth to a port then it is capable of, the buffers will fill up and eventually packets will be dropped, but providing multiple 1G ports is not inherently unreasonable.

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  • \$\begingroup\$ Thank you for the answer. Could you please explain a little more with an image and a bit more clearly, please. Not able to follow when you said, copper ports are forwarding 500Mbps to "Each other" and 500Mbps to the GMII Port. ? \$\endgroup\$
    – user220456
    Commented Apr 20, 2022 at 16:36
  • \$\begingroup\$ In that context, how can we say the switch IC is non-blocking as claimed by the datasheet? \$\endgroup\$
    – user220456
    Commented Apr 20, 2022 at 16:36
  • \$\begingroup\$ @Newbie you need to simplify your thinking. There is no magic happening here. Think of the switch like a box sorting facility like FedEx has. In this description, the boxes are ethernet frames. The facility can sort as many boxes you can throw at it (non-blocking) but if there are not enough trucks to take the sorted boxes away, then they will be discarded. If all the boxes were going to Alberqueque then you would have problems. The reality is that in most cases not all the boxes are going to the same place. \$\endgroup\$
    – Kartman
    Commented Apr 20, 2022 at 17:23

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