There is an obligation to visit ones Rebbe on holidays (see here for details)
However the mitzvah or obligation that we know of, what does this mean exactly? Is it enough if I "see" my Rebbe even if he doesn't see me? What about if he sees me as well? Do I need to speak to him specifically? Say 'shalom', speak in learning? What's the definition of kabbalos pnei rabo?