[FS#4220] MTU > 1500 settings are missing for WAN and LANx mt7621

[OpenWrt Bugs]

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A new Flyspray task has been opened.  Details are below. 

User who did this - nicefile (nicefile) 

Attached to Project - OpenWrt/LEDE Project
Summary - MTU > 1500 settings are missing for WAN and LANx mt7621
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Very Low
Reported Version - Trunk
Due in Version - Undecided
Due Date - Undecided
Details -  Dir878 /882 /882-r1 (mt7530)
 OpenWrt SNAPSHOT, r18502-53b87a7a28 but  seems linux 5.10 builds are afected as 5.4 kernel it's fine.

There is issue with setting MTU to 1508 for DSA driver to be rfc4638 compatibile

root at OpenWrt:~# ifconfig br-lan mtu 1508
root at OpenWrt:~# ifconfig eth0 mtu 1508
root at OpenWrt:~# ifconfig LAN2 mtu 1508
ifconfig: SIOCSIFMTU: Invalid argument

But even then MTU > 1500 seem to work for lan side.

root at OpenWrt:~# fping -s -M -b 1473 192.168.1.3
192.168.1.3 is alive

But to be able to use rfc4638 pppoe WAN must be set to 1508
root at OpenWrt:~# ifconfig wan mtu 1508
ifconfig: SIOCSIFMTU: Invalid argument

PPPoE session is always connecting  with MTU 1492



More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=4220

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