Teach lagg(4) to accept ioctl(SIOCSIFMTU) and change the MTU on all the component interfaces. If for any reason a component interface's MTU cannot be changed as requested, all component interfaces are returned to the original MTU.
Discussed on freebsd-hackers@:
There is a LOR, but I don't know how to interpret it:
Putting rstone@ down as a reviewer, per his offer on the list.