The initial Maximum Transmit Unit (MTU) that the PPPoE major interface
entity will advertise to the remote entity.
If the value of this variable is 1 then the local PPPoE entity will
use an MTU value determined by its underlying media interface.
If the value of this variable is 2 then the local PPPoE entity will
use a value determined by the PPPoE Max-Mtu-Tag transmitted from the
client in the PADR packet. If no Max-Mtu-Tag is received, the value
defaults to a maximum of 1494.
The operational MTU is limited by the MTU of the underlying media
interface minus the PPPoE frame overhead.