Ethernet slow with lot of Send Errors while WiFi is fine (MacBook Pro Retina - 2012)

Note: I do not have an alternate USB-C Ethernet adapter to test MacBook ethernet without the dock. On my list, just haven’t bought one yet to eliminate the dock, bandwidth sharing, and see if the speed and send errors issues is specific to using the dock or any wired ethernet with the MacBook.

MacBook Pro 2018 Catalina 10.15.6 with whatever ethernet driver last installed connected with USB-C cable to Kensington SD4700P dock wired 5e ethernet to access point switch to router to Spectrum modem.

Send errors during download. None during upload.

Full 355 down / 21 up using wireless to same access point from same MacBook. Ditto full speeds using Windows 10 desktop with same USB-C cable with USB adapter to USB 3.0 front port -- full wired speeds through same equipment and network path. Same symptoms with wife's MacBook and same with dock direct connected to modem.

Found the same symptoms described here: https://www.displaylink.org/forum/showthread.php?t=65874

as is said above, the most common cause of that problem is a broken wire or a mis-wire in a cable.

In Network Utility, does it report you are you actually connecting at 100 M bits/sec speeds, or 1,000M bits/sec speeds?

The modulation technique used, and the cable requirements for the two are COMPLETELY different.

I specifically called out Network Utility because it tells the truth about your ACTUAL connection speed, not just the settings.

When you decide to connect at 1GB/sec, the modulation scheme requires that all eight conductors be present and wired according to standards. I have seen a single broken wire cause a very similar spike in Transmit Errors.

The added complexity is that you can only compare cables Gigabit-to-Gigabit. Cables that are used at only 100 M bits/sec don't need more than four conductors present.

<<I get massive Send Errors but on WiFi there are zero errors. >>

The most common cause of that problem is a cable with a broken wire or a mis-wire.

If these are home-made cables, you need to be using a standard cable diagram to assign which pin to which wire (the connection is not literally straight-through.

If attempting to run at Gigabit speeds, ALL 8 conductors must be present and properly wired. cables that work great at100 speeds will fail at higher speeds if they do not have all eight wires correctly connected.

Cat-6 cables are not required, unless you are going a very long distance (near the 100 meter limit).

Send Errors are detected by listening while talking, and your Mac is detecting it cannot "talk" without its output being garbled.

If you have ruled out the cables and ruled out the ports on your Router (by swapping them around as well) that leaves the Belkin dock as the possible source of the issue.

Although actual send-errors are caused by mistakes in bits sent, it is possible that an intermediary device that can not handle jumbo frames properly could also induce errors that are reported as send errors.

I have the exact same issue with a Kensington SD4700P dock connected using supplied USB-C cable to my MacBook Plus 2018 with Catalina 10.15.6. Ditto testing using my wife’s MacBook Air, also Catalina.

I’ve been through a couple lengthy support calls with a helpful and patient Kensington support rep. Right now, speed test is showing ‘Send Errors’ in Network Utility for the Ethernet interface via the dock via USB-C and reaching 110 Mbps down / 20.5 up out of 355 down / 21 up max (Spectrum cable).

Put a wi-fi router in the office with ethernet backhaul. 5 Ghz 80 Mhz @ 866 Mbps max 3 feet away from my MacBook Pro, get full 355 / 21 speeds from same MacBook through the same access point that the Kensington dock is plugged into.

Plug the same USB-C cable into my Windows 10 desktop in same room and download speed is full 355 Mbps, solid through the same Kensington dock. I have a hand made 5e drop to the office with this access point and might not be ideal signaling. Don't have issues but wouldn't be surprised if not 100% if using a quality cable tester. So let's rule that out.

Same exact issue with MacBook Pro but not with a Windows 10 laptop plugged directly into the router. Ok, let's do this again - Kensington dock -- direct connection to the bloody Internet without the router, Windows 10 full speed no issues. MacOS Catalina MacBook (tested both) limited throughput. All that during the call with the Kensington support rep. Drop mic.