Postado originalmente por wiki mikrotik
L2MTU indicates maximal size of the frame without MAC header that can be sent by this interface.
Starting from the RouterOS v3.25 L2MTU values can be seen in \"/interface\" menu. L2MTU support is added for all Routerboard related Ethernet interfaces, VLANs, Bridge, VPLS and wireless interfaces. Some of them support configuration of L2MTU value. All other Ethernet interfaces might indicate L2MTU only if the chip set is the same as Routerboard Ethernets.
This will allow users to check if desired setup is possible. Users will be able to utilize additional bytes for VLAN and MPLS tags, or simple increase of interface MTU to get rid of the some unnecessary fragmentation.
This table shows L2MTU supported by Mikrotik RouterBoards:
