How can there be knowledge of unconditioned phenomenon without any knower?

It's strongly maintained by almost all Buddhists that there is no ultimate permanent knower. They maintain Nirvana is unconditioned phenomenon. My question is then, who knows there is an existence of such a phenomenon? One has to know or experience such a phenomenon to know about its existence. For instance, Gautama Buddha said - > There is, bhikkhus, that base (ayatana) where there is no earth, no water, no fire, no air; no base consisting of the infinity of space, no base consisting of the infinity of consciousness, no base consisting of nothingness, no base consisting of neither-perception-nor-non-perception; neither this world nor another world nor both; neither sun nor moon. Here, bhikkhus, I say there is no coming, no going, no staying, no deceasing, no uprising. Not fixed, not movable, **it has no support**. Just this is the end of suffering. Ud 8.1 Now, how Gautama Buddha knew there is such base/ayantana with those qualities of negations? To claim so one has to either experience so or accept the authority of someone, right? or I am missing something? Moreover, What is 'it' in 'it has no support'? It seems some existing substance to me which has been experienced/known thus proving the existence of some known entity.