All MTU configuration nodes contain a default value of 1500. This does not seem to be right, because those values override MTU that can be set from other sources - dynamically by DHCP client, for example, or even manually via custom scripts.
I think that it would be better to remove default values from there and allow using by default what a driver and Linux kernel suggest during interface initialization.
One of the obvious examples of the problem:
- Configure DHCP client on an interface.
- Get MTU via DHCP - it will be changed accordingly.
- Change any interface setting.
- MTU is back again to the default 1500 value.