On Mon, 2020-02-10 at 22:04 +0800, Ed Greshko wrote:
> The key is....
>
> Link partner advertised link modes: 10baseT/Half 10baseT/Full
> 100baseT/Half 100baseT/Full
>
> The other side is telling that it only supports 100Mb/s
Interesting. I have another machine (my NAS) connecting at 1Gbps to
another port on the same router. Unfortunately it runs an ancient
version of Debian so doesn't have ethtool.
1000baseT uses all 4 pairs in the cable, while 100baseT uses 2 pairs, so this
sounds like bad cable. Improperly terminated cables caused problems for
me when 1000baseT came in. Some patch cables had uneven pins in the modular
plugs when you look at the plug end-on. I suspect the installer used a worn
crimping tool and just checked for 100baseT connectivity. There are commercial
cable testers, but I found diagnostic software (DOS?) specific to the ethernet device
in a laptop that did a good job of detecting the bad cables and was able to borrow a
high quality crimping tool to tweak the badly crimped plugs.
A proper tester can find issues you won't find using PC software: