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I'm networking a bunch of Raspberry Pis together on a subnet as shown in my highly technical diagram below: enter image description here

I'm wondering if the red cables are supposed to be crossover cables or straight through? The blue cables I'm quite sure are supposed to be straight through cables (but please correct if I'm wrong).

Also what if I were to swap out any of the hubs for switches, would that change whether I should be using crossover vs. straight through cabling anywhere?

Thank you

Matt
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2 Answers2

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Most modern hardware – Ethernet switches and computer Ethernet interfaces – implements Auto-MDIX, automatically choosing the correct mode for each physical connection. (This is now a required part of Gigabit Ethernet, but also very common among 100 Mbps ports as well.) In other words, you can use the same type – regular straight-through cables – practically everywhere.

Ethernet hubs, however, do not have this feature. They do not participate in link negotiation and only electrically connect all the devices. (Although your devices are probably switches mislabelled as hubs, not actual hubs.)

Without this feature, two hubs or switches (just like two computers) would have needed a cross-over cable. That is, unless one switch had a dedicated 'Uplink' port, which would accept a straight-through cable instead. (Sometimes the last port had a physical toggle button for this.)

I bought super cheap hubs from China so I'm honestly not even sure whether they have that feature. Is there a way to check?

The easiest way to find out whether it works is to connect all the devices together and see whether the 'link' indicator lights turn on on both ends, and whether you can actually send/receive packets between them. If it works, that means it works. (Ethernet ports are transformer-isolated, which I think means that you won't physically damage anything by just using the wrong kind of cable.)

Other indications:

  • If they are 1 Gbps switches, then at least the 1G ports will always have Auto-MDIX.
  • If the specs mention "Auto-Uplink", that's just another name for Auto-MDIX, so you're good.
  • If the specs mention "store and forward", that at least means you have a switch.

Note: do not buy actual hubs. You won't get any reasonable performance out of a hub (getting worse the more devices you have), and you will never have 1 Gbps support in a hub. (And they're not any cheaper than switches anymore.)

That said, what you bought is quite likely to be switches mislabeled as hubs, but be careful regardless. I mean, "super cheap" usually contradicts "permanent and reliable".

grawity
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Technically speaking, cross over cables are supposed to be used when connecting devices of the same “type” together. Like connecting 2 PCs together, or 2 switches together.

When connecting devices of different types together you use a straight through cable. Like when plugging a PC into a switch.

There are really only two different “types” of devices. A NIC in PC configuration and a NIC in router/switch/hub configuration.

For network communications to occur the transmit line on one end needs to connect to the receive line on the other end and vice-versa. The two types of devices use opposite transmit/receive pinout configurations and a straight-through cable is used to connect them. But when connecting the same interface type together a cross-over cable is necessary.

However, that is all technically speaking. In reality, Auto MDI-X has existed for 20 years and is included pretty much on every network interface that exists nowadays. The need for a cross-over cable is very rare, and is not likely necessary in your situation. This feature may also be called Auto uplink and trade, Universal Cable Recognition and Auto Sensing.

Appleoddity
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