Wouldn't it be necessary, to actually have formally have asked a teacher to be your root guru before it is so. I mean, that there is a formal and conscious relationship between the teacher and the student. I wouldn't want to break heavy samaya by slandering or having wrong views about my root guru without actually consciously knowing he is my root guru.
Whenever a guru gives a major empowerment or direct introduction, there is always a possibility that one or more students will have an authentic discovery of their true state. He already accepts that some students their may discover their real nature, and that they might come to regard him or her as their root guru. That is what it means to take responsibility for giving transmission. When someone gives transmission they are doing so in order to help people become realized, to realized their own nature. When that happens, that guru becomes the root guru for that student, and from then on that student has real knowledge of their own nature. The person who removes that doubt for you is your root guru and no other. It also does not mean you realize that right away. Sometimes it may take you a little while to figure out. So there is never any need to ask formal permission from some Lama "Oh, can I consider you my root teacher"-- if they say yes, than what shall you do? If you do this, it means you do not have real knowledge of your state or that you have some doubts or are in a state of confusion. Especially if the request is based on just some sort of love-sick emotion that poses as faith (which happens a lot, and why unstable western students often leave this or that lama when the infatuation wears off). The identity of your root guru is based on your own knowledge. No one can tell you who your root guru is, not even some Lama who is acting as a guru. If some Lama declares to you "I am your root guru" before you have some real knowledge of the teachings, then be careful, especially if you have never received some sort of empowerment or direct introduction from that person. If you take a major empowerment from some guru, as I have said, according to the system of Sakya, etc., this person already is your root guru. In this system, one can therefore have as many root gurus as one has received major empowerments. Receiving the four empowerments automatically makes them your root guru. If you received sixteen major empowerments from sixteen different gurus you have sixteen root gurus. According to this system, however, there is also the concept of the karmic link root guru which resembles the Kagyu and Nyingma approach i.e. based on this guru’s instructions you have an authentic realization of the nature of your mind. So, if you are approaching this from a Kagyu and Dzogchen point of view, and you do not have real confidence in your state, then you don't have to consider anyone your root guru. Of course you still have gurus -- but when you have real conviction and knowledge then those people who have given you that conviction and knowledge are your actual root gurus. Before that time, they are just your gurus."