Ubuntu Version:
25.10 Server
Problem Description:
Welcome 
I have computer, NUC13 (i5, 64GB ram) with Ubuntu Server 24.04, equipped with two i226-V NICs (one built in, another one is NUC add-on). I observed some network issues, so I tested it with iPerf3. Results are very poor. iperf3 -c x.x.x.x results in speeds around 10-15-30Mbps, while iperf3 -c x.x.x.x -R shows correct transfers (up to 2.5Gbps). It may be worth to add, that UDP test iperf3 -c x.x.x.x -u -b 1000M succeed with 1Gbps. Problem exists on both NICs. I set up iperf binding to specific interfaces to isolate them during test without success. They behave exactly the same.
I already tried disabling eee, changing qdisc, using HWE kernel 6.14 and finally - upgraded Ubuntu to 25.10 with kernel 6.17. Tried messing with MTU 9000, disabled ASPM in BIOS - no success. Nothing can fix my situation. At the end of the day I removed second network interface from machine. Results looks like this:
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 1.75 MBytes 14.7 Mbits/sec
[ 5] 1.00-2.00 sec 896 KBytes 7.33 Mbits/sec
[ 5] 2.00-3.00 sec 640 KBytes 5.25 Mbits/sec
[ 5] 3.00-4.00 sec 896 KBytes 7.33 Mbits/sec
[ 5] 4.00-5.00 sec 512 KBytes 4.19 Mbits/sec
[ 5] 5.00-6.00 sec 768 KBytes 6.29 Mbits/sec
[ 5] 6.00-7.00 sec 512 KBytes 4.20 Mbits/sec
[ 5] 7.00-8.01 sec 896 KBytes 7.31 Mbits/sec
[ 5] 8.01-9.00 sec 640 KBytes 5.27 Mbits/sec
[ 5] 9.00-10.00 sec 896 KBytes 7.33 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate
[ 5] 0.00-10.00 sec 8.25 MBytes 6.92 Mbits/sec sender
[ 5] 0.00-10.00 sec 8.12 MBytes 6.81 Mbits/sec receiver
iperf Done.
And reverse iperf3 (wireless connection in this example)
Reverse mode, remote host x.x.x.x is sending
[ 5] local y.y.y.y port 60412 connected to x.x.x.x port 5201
[ ID] Interval Transfer Bitrate
[ 5] 0.00-1.00 sec 89.8 MBytes 752 Mbits/sec
[ 5] 1.00-2.00 sec 100 MBytes 841 Mbits/sec
[ 5] 2.00-3.00 sec 103 MBytes 862 Mbits/sec
[ 5] 3.00-4.00 sec 103 MBytes 865 Mbits/sec
[ 5] 4.00-5.00 sec 103 MBytes 864 Mbits/sec
[ 5] 5.00-6.00 sec 104 MBytes 875 Mbits/sec
[ 5] 6.00-7.00 sec 104 MBytes 873 Mbits/sec
[ 5] 7.00-8.00 sec 103 MBytes 865 Mbits/sec
[ 5] 8.00-9.00 sec 104 MBytes 872 Mbits/sec
[ 5] 9.00-10.01 sec 104 MBytes 866 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.01 sec 1022 MBytes 856 Mbits/sec 165 sender
[ 5] 0.00-10.01 sec 1018 MBytes 854 Mbits/sec receiver
iperf Done.
Im pretty much sure that networking itself is fine, as I have another machine connected to same switch which don’t have any problems with network saturation. Also changed cables. Its also not iperf3 issue, because openspeedtest instance shows the same problem.
ethtool output looks like this
Settings for enp86s0:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseT/Full
Supported pause frame use: Symmetric
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
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: 2500Mb/s
Duplex: Full
Auto-negotiation: on
Port: Twisted Pair
PHYAD: 0
Transceiver: internal
MDI-X: off (auto)
Supports Wake-on: pumbg
Wake-on: g
Current message level: 0x00000007 (7)
drv probe link
Link detected: yes
Any ideas/advices can be helpful.
Intel NUC 13 with i226-V NIC