Hello Izuone
An interface that has the L2 MTU set to 1000 won’t accept any frame larger than 1000 bytes. Incoming frames cannot be fragmented. In such a situation, an interface will accept the frame if it is 1000 bytes or smaller, and will drop it if it is 1001 bytes or larger. This is the behaviour will be seen for incoming and outgoing frames.
Note that it is the IP MTU configuration that tells the device if and when to fragment IP packets. This value should be set correctly so that the router will know when to fragment packets in order to accommodate any L2 MTU settings of the interfaces used.
I hope this has been helpful!
Laz