RB 750
routeros 5.5
According to this:
http://wiki.mikrotik.com/wiki/Manual:Maximum_Transmission_Unit_on_RouterBoards#MAC.2FLayer-2.2FL2_MTU
RB750 have more than 2k mtu on all interfaces.
/interface ethernet
set 0 l2mtu=1600
failed to update l2mtu
set 1 l2mtu=1600
failed to update l2mtu
Did i miss something else in global config or ?