Network speed negotiation issue

Ask Fedora
1

I have a motherboard with two Ethernet ports: enp34s0, which has a max speed of 1 Gbps, and enp42s0, which has a max speed of 2.5 Gbps. I have a networked printer in my office. The printer, which has a max speed of 100 Mbps, is connected to enp34s0, and enp42s0 is connected to my ISPs router, which has a max speed of 1 Gbps. I set up a network bridge so the rest of my family can reach the printer. So enp34s0 negotiates a speed of 100 Mbps and enp42s0 negotiates a speed of 1 Gbps.

Or at least, that is what happened up until last Saturday. Friday night, I ran dnf upgrade and shut down as usual. Saturday, I disconnected everything, rearranged the furniture in my office, moved the computer to a new spot, reconnected everything and booted up. And now enp34s0 and enp42s0 both negotiate a speed of 100 Mbps. I unplugged and replugged enp42s0, just to be sure it is plugged in solidly. I tried manually setting the link speed on enp42s0 to 1 Gbps, but that just generated errors telling me the link speed was too high and the network did not function. This is the output of ethtool enp42s0:

Settings for enp42s0:
	Supported ports: [ TP	 MII ]
	Supported link modes:   10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	                        2500baseT/Full
	Supported pause frame use: Symmetric Receive-only
	Supports auto-negotiation: Yes
	Supported FEC modes: Not reported
	Advertised link modes:  10baseT/Half 10baseT/Full
	                        100baseT/Half 100baseT/Full
	                        1000baseT/Full
	                        2500baseT/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
	Link partner advertised pause frame use: Symmetric
	Link partner advertised auto-negotiation: Yes
	Link partner advertised FEC modes: Not reported
	Speed: 100Mb/s
	Duplex: Full
	Auto-negotiation: on
	master-slave cfg: preferred slave
	Port: Twisted Pair
	PHYAD: 0
	Transceiver: internal
	MDI-X: Unknown
	Supports Wake-on: pumbg
	Wake-on: d
	Link detected: yes

I see that the link partner is advertising a max of 100 Mbps. I don’t understand that. My family’s other devices that are plugged into the ISP router are all still operating at 1 Gbps, and I made no changes to the router, which is in a different room and was not involved in the office rearrangement.

This is an x86_64 Fedora 43 Workstation machine. Between booting up on Friday and Saturday, I installed setroubleshoot and upgraded these packages (with wildcards to stand in for long lists of font and libreoffice packages):

autocorr-en-25.8.3.2-2.fc43.noarch.rpm
binutils-2.45.1-1.fc43.x86_64.rpm
clang-21.1.5-1.fc43.x86_64.rpm
clang-analyzer-21.1.5-1.fc43.x86_64.rpm
clang-libs-21.1.5-1.fc43.x86_64.rpm
clang-resource-filesystem-21.1.5-1.fc43.x86_64.rpm
clang-tools-extra-21.1.5-1.fc43.x86_64.rpm
compiler-rt-21.1.5-1.fc43.x86_64.rpm
cpp-15.2.1-4.fc43.x86_64.rpm
firewalld-2.3.2-1.fc43.noarch.rpm
firewalld-filesystem-2.3.2-1.fc43.noarch.rpm
gcc-15.2.1-4.fc43.x86_64.rpm
gcc-c++-15.2.1-4.fc43.x86_64.rpm
gcc-gdb-plugin-15.2.1-4.fc43.x86_64.rpm
gcc-gfortran-15.2.1-4.fc43.x86_64.rpm
gcc-plugin-annobin-15.2.1-4.fc43.x86_64.rpm
google-noto-*-fonts-20251101-2.fc43.noarch.rpm
google-noto-fonts-common-20251101-2.fc43.noarch.rpm
httpd-2.4.65-3.fc43.x86_64.rpm
httpd-core-2.4.65-3.fc43.x86_64.rpm
httpd-filesystem-2.4.65-3.fc43.noarch.rpm
httpd-tools-2.4.65-3.fc43.x86_64.rpm
ibus-1.5.33-1.fc43.x86_64.rpm
ibus-gtk2-1.5.33-1.fc43.x86_64.rpm
ibus-gtk3-1.5.33-1.fc43.x86_64.rpm
ibus-gtk4-1.5.33-1.fc43.x86_64.rpm
ibus-libs-1.5.33-1.fc43.x86_64.rpm
ibus-setup-1.5.33-1.fc43.noarch.rpm
libasan-15.2.1-4.fc43.x86_64.rpm
libatomic-15.2.1-4.fc43.i686.rpm
libatomic-15.2.1-4.fc43.x86_64.rpm
libcxxabi-21.1.5-1.fc43.x86_64.rpm
libgcc-15.2.1-4.fc43.i686.rpm
libgcc-15.2.1-4.fc43.x86_64.rpm
libgccjit-15.2.1-4.fc43.x86_64.rpm
libgfortran-15.2.1-4.fc43.x86_64.rpm
libgomp-15.2.1-4.fc43.i686.rpm
libgomp-15.2.1-4.fc43.x86_64.rpm
libomp-21.1.5-1.fc43.x86_64.rpm
libomp-devel-21.1.5-1.fc43.x86_64.rpm
libquadmath-15.2.1-4.fc43.x86_64.rpm
libquadmath-devel-15.2.1-4.fc43.x86_64.rpm
libreoffice-*-25.8.3.2-2.fc43.x86_64.rpm
libstdc++-15.2.1-4.fc43.x86_64.rpm
libstdc++-devel-15.2.1-4.fc43.x86_64.rpm
libtsan-15.2.1-4.fc43.x86_64.rpm
libubsan-15.2.1-4.fc43.x86_64.rpm
llvm-21.1.5-1.fc43.x86_64.rpm
llvm-filesystem-21.1.5-1.fc43.x86_64.rpm
llvm-libs-21.1.5-1.fc43.x86_64.rpm
lua-5.4.8-3.fc43.x86_64.rpm
lua-libs-5.4.8-3.fc43.x86_64.rpm
mod_lua-2.4.65-3.fc43.x86_64.rpm
python3-firewall-2.3.2-1.fc43.noarch.rpm
selinux-policy-42.15-1.fc43.noarch.rpm
selinux-policy-targeted-42.15-1.fc43.noarch.rpm

I don’t see anything on that list that seems likely to be the cause of this issue. If you were me, what would you try next?

2

I forgot to mention one critical point: did I manage to get the Ethernet cables reversed? No, I checked that. I unplugged the printer and verified that I got a log message telling me that enp34s0 was disconnected. In any case, both Ethernet ports are capable of operating at 1 Gpbs, so it shouldn’t matter even if I did.

3

Different qualities of Ethernet cables. Did you swap the cables?

4

No hardware changed. All I did was move stuff a few feet from its original position and plug it all back in. The cable between enp42s0 and the router is CAT6a. I suppose it is possible that I damaged the cable somehow, although I had it tucked back out of the way while I was moving stuff around. Hmmm…

5

Have you tried rebooting the router to make it “re-re-negotiate”? A long shot but maybe worth a go.

6

I wasn’t able to do that until today. No change. I guess I need to find a replacement cable and see if that is causing the issue. Thanks for the suggestions, everyone.