Mac Studio Ethernet Port Restricting Speeds

I am experiencing this identical problem, the only difference being that mine is a Mac Studio M1 Max. Otherwise I'm seeing exactly the same scenario you describe in your original post and your follow-ups.

My Mac Studio is connected via ethernet to a TP-Link switch which previously showed the connection as gigabit via an LED on the switch's front panel. That LED is no longer lit, and I'm seeing max speeds of ~95Mpbs. If I unplug the Mac Studio and use that same ethernet cable to connect my old 2013 iMac instead—in other words, everything downstream from that ethernet connector is 100% the same—suddenly the switch's LED again shows a gigabit connection and I see expected speeds (900+Mbps up/down) on the iMac.

I only realized there was a problem a couple of weeks ago when Dropbox syncs suddenly seemed to take much longer than normal, so I can't pinpoint exactly when the issue started or if it's connected to a particular macOS update. (I was running 14.6 when I discovered the problem, but the 14.6.1 update didn't change anything.)

Ethernet actual CLOCK 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>

Ethernet 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. ---of course usng the correct en number for YOUR interface

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     -     -

Reading the top line, 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)

how did you determine the actual speed?

PSC6060 wrote:

On my Apple M1 Ultra I have had a 10gb/s cat 6 connection to my server that has always provided full speed. Suddenly that same connection is being capped at 100mb/s, making it pretty much unusable for our needs.

The 10 Gigabit Ethernet ports on Mac Studios do not support operation at speeds below 1 Gigabit per second. If yours is making a connection at Fast Ethernet (100 Mbps) speeds, something is very unusual.

100 M bits/sec Ethernet uses baseband signaling (one bit per signalling interval). Everything faster uses a modulated signal. As Servant of Cats stated, Mac Studio has dropped Ethernet baseband signaling altogether, and sees not support 100 M Bits/sec speeds on its built-in Ethernet port at all.

I can not resolve what your Switch is telling you with what Mac Studio built-in Ethernet actually supports. But I do know that if all eight conductors are not wired correctly, Gigabit Ethernet and faster can behave VERY strangely. (100 M bits/sec ethernet only uses four of the eight wires.)

A very common 'internet speed test' confusion is that the Mac uses the TOPMOST network interface listed at Settings/Preferences > Network for connection to the Internet. If Wi-Fi is topmost, that is what an "internet speed test' will measure, regardless of what you intended..

Just to double-check … everything that we have been talking about is in units of bits per second? Not bytes?

Standard convention for network interfaces is to quote nominal speed in BITS per second. The same rule goes for interfaces like USB, FireWire, and Thunderbolt. But when people benchmark of read and write speeds of external drives, you will often see the results presented in terms of megaBYTES per second.

Running your tests above, I get the following results:

media: autoselect (100baseTX <full-duplex,flow-control>)

Name    Mtu  Network    Address      Ipkts Ierrs  Opkts Oerrs Coll
en0    1500 <Link#20>  9c:76:0e:49:18:9f  102166   0    0   0   0
en0    1500 mac-studio. fe80:14::1027:960  102166   -    0   -   -
en0    1500 192.168.138  mac-studio.lan  102166   -    0   -   -

Note the "autoselect" configuration above. If I manually set it to 1000baseT (or any higher option), I get the following:

media: 1000baseT <full-duplex> (100baseTX <full-duplex>)

Name    Mtu  Network    Address      Ipkts Ierrs  Opkts Oerrs Coll
en0    1500 <Link#20>  9c:76:0e:49:18:9f  106681   0    0   0   0
en0    1500 mac-studio. fe80:14::1027:960  106681   -    0   -   -
en0    1500 192.168.138  mac-studio.lan  106681   -    0   -   -

Hmmm, just wondering what it says about MTU & Duplex?

Or could Low Data Mode have gotten selected?

Nice work.