Unlike the other Natural attacks, the text for the Trample Evolution (page 62) does not mention if this is a Primary or a Secondary attack...!
This should be clarified.
Trample (Ex): An eidolon gains the ability to crush its foes underfoot, gaining the trample ability. As a full-round action, the eidolon can overrun any creature that is at least one size smaller than itself. This works like the overrun combat maneuver, but the eidolon does not need to make a check, it merely has to move over opponents in its path. The creatures take 1d6 points of damage (1d8 if Large, 2d6 if Huge), plus 1-1/2 times the eidolon’s Strength modifier. Targets of the trample can make attacks of opportunity at a –4 penalty. If a target forgoes the attack of opportunity, it can make a Reflex save for half damage. The DC of this save is 10 + 1/2 the eidolon’s HD + the eidolon’s Strength modifier. A trampling eidolon can only deal trampling damage to a creature once per round. This evolution is only available to eidolons of the biped or quadruped base forms.
It's neither. It is a special attack, performed on it's own as a Full-round action. It's the same as the Trample ability from the Universal Monster Rules