Tiebreaking
If the aggregate score is tied after the two legs, various methods can be used to break ties. Under the away goals rule, the team who scored more away goals advances. If away goals are equal, or are not considered, then the tie may be decided by extra time and/or penalty shootout. Replays, at the second-leg venue or a neutral venue, were formerly used in European club competitions. In the Liguilla (playoffs) of the Primera División de México, the team with the better regular-season record advances. In the promotion playoffs in Italy's Serie B (which do not necessarily occur in a given season), two-legged ties that are level on aggregate at the end of regulation time of the second leg go to extra time (away goals are not used); if the tie remains level after extra time, the team that finished higher on the league table advances.
Read more about this topic: Two-legged Tie