The message means that the MTU cannot be configured for the datalink for the specific hardware in question.
If you are trying to set the MTU to support MPLS label stack,
1) If you have not configured MPLS MTU, then any 1500 bytes packet would be fragmented at 1492 and the resulting fragments would be assigned the labels. As the MPLS MTU defaults to the datalink MTU value.
2) Most ethernet hardware is supposed to switch 4 to 8 bytes more than the conventional 1500 bytes frame size. If this support is there in the hardware being used by you then, you can configure MPLS MTU to avoid fragmentation at 1492, and transmit the 1500 bytes packet. Hence supporting the dual label stack.
For further information on the MTU, as a reading reference, you can refer to RFC 3032 complete Section "3".
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