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1Gbps Ethernet drops to 100Mbps

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Hi all,

I have a 1Gbps network port that correctly connects as 1Gbps full duplex 
on boot, then drops to 100Mbps 4 seconds later.

Forcing 1Gbps full duplex with ethtool works fine. Tested to make sure 
the actual speed is over 100Mbps.

I am trying to determine what causes the drop to 100Mbps and how to 
avoid it.

The network port is on a Thunderbolt 3 hub in case this matters. Though 
it appears that it looks just like a regular 1Gbps card to igb driver.

Tried two different Cat6 cables at 5 ft distance to the switch.

There is nothing aside from loopback in /etc/network/interfaces and 
/etc/network/interfaces.d.


Relevant log entries:

# dmesg | grep -E '(igb|eth|enp9s0)'
[    1.707669] wmi_bus wmi_bus-PNP0C14:01: WQBC data block query control 
method not found
[    5.578360] igb: Intel(R) Gigabit Ethernet Network Driver - version 
5.4.0-k
[    5.578360] igb: Copyright (c) 2007-2014 Intel Corporation.
[    6.351902] igb 0000:09:00.0: added PHC on eth0
[    6.351903] igb 0000:09:00.0: Intel(R) Gigabit Ethernet Network 
Connection
[    6.351904] igb 0000:09:00.0: eth0: (PCIe:2.5Gb/s:Width x1) 
64:4b:f0:10:5e:80
[    6.352004] igb 0000:09:00.0: eth0: PBA No: 000300-000
[    6.352005] igb 0000:09:00.0: Using MSI-X interrupts. 4 rx queue(s), 
4 tx queue(s)
[    6.352464] igb 0000:09:00.0 enp9s0: renamed from eth0
[   71.234244] IPv6: ADDRCONF(NETDEV_UP): enp9s0: link is not ready
[   71.262391] IPv6: ADDRCONF(NETDEV_UP): enp9s0: link is not ready
[   74.272257] igb 0000:09:00.0 enp9s0: igb: enp9s0 NIC Link is Up 1000 
Mbps Full Duplex, Flow Control: RX/TX
[   74.487967] IPv6: ADDRCONF(NETDEV_CHANGE): enp9s0: link becomes ready
[   78.623864] igb 0000:09:00.0 enp9s0: igb: enp9s0 NIC Link is Up 100 
Mbps Half Duplex, Flow Control: None
[   78.623869] igb 0000:09:00.0: EEE Disabled: unsupported at half 
duplex. Re-enable using ethtool when at full duplex.


Then forcing with ehttool:

# ethtool -s enp9s0 speed 1000 duplex full

# dmesg | grep -E '(igb|eth|enp9s0)'
...
[  346.656896] igb 0000:09:00.0 enp9s0: igb: enp9s0 NIC Link is Up 1000 
Mbps Full Duplex, Flow Control: RX/TX


Any ideas would be much appreciated. Thanks.

-- 
Nazar

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Thread

1Gbps Ethernet drops to 100Mbps Nazar Zhuk <nazar@zhuk.online> - 2020-05-13 21:00 +0200
  Re: 1Gbps Ethernet drops to 100Mbps Andy Smith <andy@strugglers.net> - 2020-05-14 08:00 +0200
  Re: 1Gbps Ethernet drops to 100Mbps Michael Stone <mstone@debian.org> - 2020-05-14 14:00 +0200
  Re: 1Gbps Ethernet drops to 100Mbps Nazar Zhuk <nazar@zhuk.online> - 2020-05-19 21:40 +0200

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