Why is my laptop (Debian 10) only connecting with 100Mbit/s instead of Gigabit?

My laptop (Thinkpad T450s) with Debian 10 only connects with 100Mbit/s instead of Gigabit through ethernet, although the network card should be capable of Gigabit LAN (see #2 below). I tried a machine running Ubuntu 18.04 on the same cable (see #5 below) and it got Gigabit.

If I manually set link speed to 1000Mbit/s, nothing changes. If I enable auto-negotiation, it won't even connect and after disabling auto-negotiation again it can't connect anymore. I then have to delete the ethernet-connection in the network manager GUI and create a new one with auto-negotiation disabled.

• According to Wikipedia, auto-negotiation is required for Gigabit LAN, so I suspect auto-negotiation somehow failing might be the root of the problem. Then again, auto-negotiation seems to have worked from a live Ubuntu 18.04 on the same machine, in the sense that I had internet, but with a link speed of 100 Mbit/s, so I'm unsure at this point.

How do I configure my network settings correctly in order to use Gigabit LAN?

These are the diagnostic steps I've run so far:

1. sudo ethtool enp0s25:

   Settings for enp0s25:
     Supported ports:              [ TP ]
     Supported link modes:         10baseT/Half 10baseT/Full
                                   100baseT/Half 100baseT/Full
                                   1000baseT/Full
     Supported pause frame use:    No
     Supports auto-negotiation:    Yes
     Supported FEC modes:          Not reported
     Advertised link modes:        Not reported
     Advertised pause frame use:   No
     Advertised auto-negotiation:  No
     Advertised FEC modes:         Not reported
   
     Speed:                  100Mb/s
     Duplex:                 Full
     Port:                   Twisted Pair
     PHYAD:                  1
     Transceiver:            internal
     Auto-negotiation:       off
     MDI-X:                  on (auto)
     Supports Wake-on:       pumbg
     Wake-on:                g
     Current message level:  0x00000007 (7)
                             drv probe link
     Link detected:          yes
   

2. lscpi:

   00:19.0 Ethernet controller: Intel Corporation Ethernet Connection (3) I218-LM (rev 03)
   

   According to Intel, the card supports Gigabit LAN (see here) and there seems to be no firmware missing. I checked with dmesg | grep -i firm and it says (Bluetooth edited out):

   [254037.281327]  (NULL device *):  firmware: direct-loading firmware intel/ibt-hw-37.8.10-fw-1.10.3.11.e.bseq
   [254037.281525]  (NULL device *):  firmware: direct-loading firmware iwlwifi-7265D-29.ucode
   [255529.699990]  (NULL device *):  firmware: direct-loading firmware iwlwifi-7265D-29.ucode
   [259054.226196]  (NULL device *):  firmware: direct-loading firmware intel/ibt-hw-37.8.10-fw-1.10.3.11.e.bseq
   [259054.229879]  (NULL device *):  firmware: direct-loading firmware iwlwifi-7265D-29.ucode
   [269591.798078]  (NULL device *):  firmware: direct-loading firmware iwlwifi-7265D-29.ucode
   

3. sudo lspci -k -nn -s 00:19.0:

   00:19.0 Ethernet controller [0200]:
     Intel Corporation Ethernet Connection (3) I218-LM [8086:15a2] (rev 03)
       Subsystem:              Lenovo Ethernet Connection (3) I218-LM [17aa:2226]
       Kernel driver in use:   e1000e
       Kernel modules:         e1000e
   

   sudo modinfo e1000e | grep -e ^filename -e ^vermagic -e ^parm:

   filename:   /lib/modules/4.19.0-8-amd64/kernel/drivers/net/ethernet/intel/e1000e/e1000e.ko
   vermagic:   4.19.0-8-amd64 SMP mod_unload modversions
   
   parm:   debug:Debug level (0=none,...,16=all) (int)
   parm:   copybreak:Maximum size of packet that is copied to a new buffer on receive (uint)
   parm:   TxIntDelay:Transmit Interrupt Delay (array of int)
   parm:   TxAbsIntDelay:Transmit Absolute Interrupt Delay (array of int)
   parm:   RxIntDelay:Receive Interrupt Delay (array of int)
   parm:   RxAbsIntDelay:Receive Absolute Interrupt Delay (array of int)
   parm:   InterruptThrottleRate:Interrupt Throttling Rate (array of int)
   parm:   IntMode:Interrupt Mode (array of int)
   parm:   SmartPowerDownEnable:Enable PHY smart power down (array of int)
   parm:   KumeranLockLoss:Enable Kumeran lock loss workaround (array of int)
   parm:   WriteProtectNVM:Write-protect NVM [WARNING: disabling this can lead to corrupted NVM] (array of int)
   parm:   CrcStripping:Enable CRC Stripping, disable if your BMC needs the CRC (array of int)
   

4. I booted Ubuntu 18.04 from a LiveUSB and ran the same speed test, receiving 90/40mbps down/up. ethtool reported link speed 100Mbit/s with auto-negotiation enabled (by default, not manually set by me), and sudo ethtool -s enp0s25 speed 1000 reported cannot advertise link speed 1000

5. I used the same cable (15m CAT6) on another machine, running Ubuntu 18.04 with auto-negotiation enabled, and Gigabit worked (confirmed it with a speed test). LED on the access point was flashing green, instead of yellow, as is the case with my machine. The report of the speed test was 250/84Mbps down/up on the Ubuntu machine vs 90/40Mbps down/up on the Debian one.
   
   ethtool: (Ubuntu machine)

   Settings for enx00e04c6845e0:
     Supported ports:              [ TP MII ]
     Supported link modes:         10baseT/Half 10baseT/Full
                                   100baseT/Half 100baseT/Full
                                   1000baseT/Half 1000baseT/Full
     Supported pause frame use:    No
     Supports auto-negotiation:    Yes
     Supported FEC modes:          Not reported
     Advertised link modes:        10baseT/Half 10baseT/Full
                                   100baseT/Half 100baseT/Full
                                   1000baseT/Full
     Advertised pause frame use:   Symmetric Receive-only
     Advertised auto-negotiation:  Yes
     Advertised FEC modes:         Not reported
   
     Link partner advertised link modes:         10baseT/Half 10baseT/Full
                                                 100baseT/Half 100baseT/Full
                                                 1000baseT/Full
     Link partner advertised pause frame use:    Symmetric
     Link partner advertised auto-negotiation:   Yes
     Link partner advertised FEC modes:          Not reported
   
     Speed:                  1000Mb/s
     Duplex:                 Full
     Port:                   MII
     PHYAD:                  32
     Transceiver:            internal
     Auto-negotiation:       on
     Supports Wake-on:       pumbg
     Wake-on:                d
     Current message level:  0x00007fff (32767)
                             drv probe link timer ifdown ifup rx_err tx_err tx_queued intr tx_done rx_status pktdata hw wol
     Link detected:          yes
   

   • Note: the whole set of options starting with Link partner advertised is missing from my output; Is it possible my machine doesn't understand the advertisement of auto-negotiation from the AP?
6. Someone pointed out that Green Ethernet or Energy Efficient Ethernet (eee) might be the problem, so I tried disabling it with ethtool --set-eee eth0 eee off, as described here, but nothing changed.