According to the N+ Manual, MTU value can only be changed when using PPPoE WAN setup(DSL in most cases) so assuming you use cable with MAC auth DHCP then the default MTU value of 1500 will be used. Such an modern router not having an manual setting for MTU under DHCP, henders those that upgrade to services like D3 and FiOS, because they most likely require larger MTU values for peak performance.