Faster-than-Gigabit Ethernet:
"energy efficient" drops power to the 10Gb Ethernet chip to save energy. It is NOT compatible with Top Speed. In the hardware pane, under Duplex, set: “Full-Duplex, Flow Control” NOT “Full-Duplex, Flow Control, power efficient” to disable power saving and boost top speed.
Speeds faster than 100 M bit/sec Ethernet are NOT compatible with cables that have fewer than all four pairs available for data.
If you have some fancy equipment at the other end of the cable, it is possible it is trying to make a 10Gb connection. A 10Gb (or 5Gb or 2.5Gb) connection is only stable when cables are excellent and fairly short (like Category-6 rated cables under 100 feet). If either of those are not true, or you have you added patch cables that are not Category-6 rated, you could be seeing it connect at a faster-than-Gigabit speed, then error out and disconnect.
The setting [√] ABV/AEV mode selected on the hardware pane of advanced Wi-Fi Ethernet OR in your Router may constrain the connection in unexpected ways that could limit overall performance. The feature is used ONLY for prioritizing video streams.
Actual Speed:
The good way to check the actual connection speed USED to be Network Utility, But in Catalina and later, Apple has deprecated Network Utility and now you have to use a Terminal command to see your actual connection speed. First, you need to know what en number the link is. then you use a command like this one, substituting the actual en number.
my main Ethernet connection uses BSD name en2 (as shown in) :
menu > about this Mac > (system report) > network:
Aquantia AQC107-B0:
Name: ethernet
Type: Ethernet Controller
Bus: PCI
Slot: Slot-3
Vendor ID: 0x1d6a
Device ID: 0x87b1
Subsystem Vendor ID: 0x1d6a
Subsystem ID: 0x0001
Revision ID: 0x0002
Link Width: x4
BSD name: en2
Kext name: AppleEthernetAquantiaAqtion.kext
Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleEthernetAquantiaAqtion.kext
Version: 1.0.64
Terminal command:
ifconfig en2 | grep media
with this as my output for 10 Gigabit Ethernet:
media: 10Gbase-T <full-duplex,flow-control>
For ‘regular’ Gigabit Ethernet, you should get this instead:
media: 1000baseT <full-duplex,flow-control>
Errors detected:
To see if an Ethernet link is throwing more than a handful of initial errors, you can use Terminal command:
netstat -I en2
This is the resulting output. Counters are In-packets, In-errors, Out-packets, Out-Errors, Collisions. There should never be more than handful of errors from starting up, and in most cases, NONE.
Name Mtu Network Address Ipkts Ierrs Opkts Oerrs Coll
en2 8163 <Link#4> 00:01:d2:1a:00:dd 696697 0 484301 0 0
en2 8163 grantsmacpr fe80:4::461:ea0d: 696697 - 484301 - -
en2 8163 192.168.0/23 192.168.0.204 696697 - 484301 - -
If the link were running beyond its ability to run and be stable, for example it auto-speeded to 10Gb but the cabling could only reliably support 2.5Gb, we would see non-zero errors counts, and errors increasing over time. (and possibly, disconnecting)
