After buying a new central switch for my home network I noticed that a smaller gigabit switch in one of our rooms connected with only 100 Mbps. Because I had a similar problem recently where my laptop (which we now used for like 10 years!) connected with 1 Gbps to the central switch without problems, our brand new laptop did so only with 100 Mbps. While re-crimping the respective ethernet outlet fixed that problem, doing the same with the other ethernet outlet did not have the desired effect. As it turned out, the problem was the cable connecting the outlet with the switch! It was an UTP (unshielded twister pair) cable. After replacing it with an STP (shielded twister pair) cable, the switch connected with 1 Gbps.
I was also testing my internet speed recently and was very annoyed that I only ever got like a maximum of 100 Mbps down instead of the 125 Mbps which I pay my service provider for (not that I would notice the difference if I wouldn't measure it
). I complained to the service provider and was informed that my modem was working flawlessly and that my signal was optimal. I was instructed to take measurements directly at the modem, which I did and voilà: 125 Mbps. Because of personal paranoia I have another router sitting in between my service providers modem (it's actually a router itself) and my home network. My service providers modem was connected to my router with an UTP cable!
Interestingly, I did not find any reference on the internet stating that UTP cables would only work up to 100 Mbps.