Edit /etc/network/interfaces, and change it to this...
auto lo
iface lo inet loopback
Your ethernet is connecting at 100Mb, but it's capable of 1Gb. This is either a cable problem (most common), or a switch/hub/router that's not capable of gigabit connections. Are you using cat 5e or cat 6 cables?
Update #1:
lspci -nn | grep -i ethernet = 02:00.0 Ethernet controller [0200]: Realtek Semiconductor Co., Ltd. RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller [10ec:8168] (rev 0c)
MSI/MSIX interrupts were enabled for certain ethernet cards in Ubuntu 20.xx. This can cause intermittent ethernet operation. Here's a patch to fix it. Follow the embedded instructions to install.
#!/bin/sh
# https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1779817
#
# Attached is a work-around for the in-kernel driver that is as unhacky as I can make it.
# filename: r8169_disable_msi
# Drop it in /etc/initramfs-tools/scripts/init-top and chmod a+x it. Add 'r8169_disable_msi'
# to your kernel command line (/etc/default/grub, GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
# usually.)
# Remember to update-initramfs and update-grub as necessary.
# sudo update-initramfs -c -k $(uname -r)
# sudo update-grub
# reboot
# For the moment it disables MSI on everything with the ID 0x10ec:0x8168, as there seems to
# be no way to get the MAC version from userspace - and certainly not before the driver is
# loaded. Other PCI IDs may need adding..
# Still hoping we can cherry pick the in-driver workaround for bionic...?
PREREQ=""
prereqs()
{
echo "$PREREQ"
}
case $1 in
# get pre-requisites
prereqs)
prereqs
exit 0
;;
esac
disable_msi () {
for i in /sys/bus/pci/devices/*; do
if [ $(cat $i/vendor) = "0x10ec" -a $(cat $i/device) = "0x8168" ]; then
echo 0 >$i/msi_bus
fi
done
}
for x in $(cat /proc/cmdline); do
case ${x} in
r8169_disable_msi)
disable_msi
break
;;
esac
done
Update #2:
The patch is working, but the ethernet is still failing, because the symlinks in /boot are wrong. See below...
Update #3:
cd /boot # change directory
ls -al # get current listing
sudo rm -i initrd.img # delete bad symlink
sudo rm -i vmlinuz # delete bad symlink
sudo mv initrd.img.old initrd.img # rename remaining symlink
sudo mv vmlinuz.old vmlinuz # rename remaining symlink
sudo ln -s initrd.img-5.8.0-43-generic initrd.img.old # recreate symlink
sudo ln -s vmlinuz-5.8.0-43-generic vmlinuz.old # recreate symlink
ls -al # compare new symlinks to original listing
reboot # reboot and verify ethernet operation
