The maximum legal length of a frame specified in IEEE 802.3-2008 is 1,518 bytes for non-VLAN tagged frames. VLAN tagged frames can be extended to 1,522 bytes. When jumbo frame handling is disabled and you attempt to transmit a frame that exceeds the maximum legal length, the MAC core inserts an error code to corrupt the current frame and the frame is truncated to the maximum legal length. When jumbo frame handling is enabled, frames that are longer than the legal maximum are transmitted error-free.
It is also possible to specify a different maximum frame size. If this is enabled and the frame exceeds the configured value then the frame is corrupted. In this case, VLAN frames are not treated separately. If jumbo frame handling is enabled, it takes precedence and the configured value is ignored.
Packets < 64 bytes are considered undersized according to the Ethernet 802.3-2012 specification. Undersized packets might cause the core to lock up and this must be avoided.