Comment 6 for bug 1565804

In my testing on Xenial, you cannot set a child interface MTU to a value greater than its parent. For example, if eth0's MTU is 1500, setting eth0.100's MTU to 9000 (or 1501) causes:

SIOCSIFMTU: Numerical result out of range

This restriction makes sense when you consider the physical topology, since packets from eth0.100 would be sent via eth0. If the system attempts to send a packet larger than 1500 bytes via eth0 in this case, it would be dropped.

This issue makes me wish it were possible to set a "L3 MTU" (i.e., one that would be the default PMTU MSS, and fragment threshold for other packet types such as UDP and ICMP).