[OpenWrt-Devel] 18.06 Bug: Baby Jumbo Frames on mt7621

Jaap Buurman jaapbuurman at gmail.com
Thu May 24 06:00:45 EDT 2018

Dear all,

I found some additional information in the system log: Thu May 24
11:38:39 2018 kern.err kernel: [83864.729458] eth0: Invalid MTU 1508
requested, hw max 1500
Digging deeper, this seems like a message that is spawned by a
function in /net/core.dev.c of the linux kernel:

if (dev->max_mtu > 0 && new_mtu > dev->max_mtu) {
net_err_ratelimited("%s: Invalid MTU %d requested, hw max %d\n",
dev->name, new_mtu, dev->max_mtu);
return -EINVAL;

Is there anybody that happens to know where exactly this max_mtu value
is set to 1500? For mt7621 devices this should be 2048 (Baby jambo

Thank you very much in advance.

Yours sincerely,

Jaap Buurman

On Tue, May 22, 2018 at 3:05 PM, Jaap Buurman <jaapbuurman at gmail.com> wrote:
> Dear all,
> The switch to the 4.14 kernel apparently broke the baby jumbo frames
> support of 2048 bytes that the switch is capable off. I found out that
> changing the mtu above 1500 via Luci no longer applies properly.
> Trying to manually change the mtu via ssh also fails:
> root at LEDE:~# ifconfig eth0 mtu 1508 up
> ifconfig: SIOCSIFMTU: Invalid argument
> If there is any additional information that I can supply, please let
> me know. I am also more than willing to help test potential fixes :)
> Yours sincerely,
> Jaap Buurman

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