The interaction between trample and protection has changed over the
   course of the game.  Nowadays, it's as follows:

   Combat damage happens in two distinct steps, assignment and dealing.
   In assignment, a record is made of the source, destination and amount
   of each piece of combat damage.  If a creature has trample, then
   it's legal to assign some or all of the damage from that creature
   to the defending player, as long as the assignment also assigns
   lethal damage to all creatures blocking the creature with trample.
   Lethal damage is considered as however much damage is needed to make
   up the difference between damage already marked on the creature from
   before the assignment was made, and the toughness of the creature.
   Effects that would alter the amount of damage that actually gets dealt
   are not taken into account at this stage.  Note that any amount of
   damage from a creature with deathtouch is considered lethal damage.

   When the damage is dealt, there may be replacements that alter the
   amount of damage or prevent it entirely.  This is where the protection
   can step in and shield the creature.