10G SFP - iperf3 only 3Gbps

Banana Pi Router design BPI-R4/BPI-R4 Pro(MT7988)
1

Hi everyone,

I’ve run into an interesting situation and would like to hear your thoughts.

Equipment

  • ASUS GT-AX11000 Pro (10GBase-T port, directly connected to SFP+ on Banana R4)
  • Banana R4 running OpenWrt 25.12.1
  • Cable: cat6

Link check

ASUS:

Settings for eth5:
        Supported ports: [ TP ]
        Supported link modes:   100baseT/Full
                                1000baseT/Full
                                10000baseT/Full
                                2500baseT/Full
                                5000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10000baseT/Full
        Advertised pause frame use: No
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 10000Mb/s
        Duplex: Full
        Port: Twisted Pair
        PHYAD: 21
        Transceiver: internal
        Auto-negotiation: off
        MDI-X: Unknown
        Link detected: yes

Banana R4 (sfp-lan):

Settings for sfp-lan:
        Supported ports: [ FIBRE ]
        Supported link modes:   10000baseSR/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: No
        Supported FEC modes: Not reported
        Advertised link modes:  10000baseSR/Full
        Advertised pause frame use: Symmetric Receive-only
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: 10000Mb/s
        Duplex: Full
        Auto-negotiation: off
        Port: FIBRE
        PHYAD: 0
        Transceiver: internal
        Current message level: 0x000000ff (255)
                               drv probe link timer ifdown ifup rx_err tx_err
        Link detected: yes

The link is up at 10G with no errors.

iperf3 tests

All iperf3 tests were run over SSH on ASUS and Banana R4.

Single-thread TCP:

[ 4] 0.00-10.00 sec 3.98 GBytes 3.42 Gbits/sec 0 sender
[ 4] 0.00-10.00 sec 3.98 GBytes 3.42 Gbits/sec receiver

Multi-thread TCP (-P 8):

  • Result is the same — ~3.4 Gbps, CPU on both devices ~20%

MTU test:

  • Tried setting mtu 9000 on br-lan and interfaces → Jumbo frames didn’t improve speed, still the same

What’s been checked

  • CPU is not the bottleneck (~20%)
  • Link is truly 10G
  • Single-thread and multi-thread tests show the same speed
  • Jumbo frames (MTU 9000) tested → no effect

Honestly, I don’t really understand what the problem is or how to reach the coveted 9 gigabits per second ? ))

SFP stick