Flint 4 4.9.1 - Jumbo frame MTU settings above 1500 do not take effect on LAN-side ports, despite the interface reporting the configured MTU

Router
1

Configuration applied:

  • Physical SFP+ port (eth2) and VLAN sub-interface (eth1.1) both set to MTU 9014 — confirmed via ip link show on the router.

  • Bridge device br-lan set to MTU 9014 — confirmed via ifconfig.

  • Connected device: Ubuntu 24.04 server, NIC (Mellanox ConnectX-4 Lx) set to matching MTU 9014, connected directly to the SFP+ port via DAC cable, no intermediate switch.

Symptom: With MTU 9014 set on both the router (port + bridge) and the server NIC, ping -M do -s 8972 (9000-byte packet) to the router's own br-lan IP (192.168.1.1) fails with 100% packet loss. Binary search shows the actual working ceiling is exactly standard 1500-byte MTU (ping -M do -s 1472 succeeds; -s 1500 and above fails), regardless of the 9014 value shown in interface status.

Reverting the server NIC back to MTU 1500 restores normal throughput immediately, confirming the router-side jumbo frame configuration is not actually being honored at the hardware/driver level despite being accepted and displayed in software (ip link / LuCI).

Reference: this matches a previously reported issue on a Flint 2 (openwrt/openwrt#21162) describing the same symptom — jumbo frames configured on br-lan and NIC causing connectivity failure, and the WAN port being unable to exceed 1500 MTU regardless of configuration.

Request: please confirm whether jumbo frame / MTU >1500 support is planned or already available in a newer firmware build for the Flint 4, or whether this is a known hardware/driver limitation of the current MediaTek switch driver in the stock GL.iNet firmware.

1 Like
3

Hi,

Thanks for reporting this issue.
We have reproduced the behavior and we have recorded the case and forwarded it to the relevant team for further investigation.

Thank you as well for the detailed testing information you provided.

1 Like
4

Thank you!

5

Hi,

You're welcome, and thank you again for the detailed testing and information.
If there are any further updates on this issue, we will share them with you here.