How do bunkers work in Risk Legacy?
A bunker adds 1 to the highest defender's die as long as it is less than 6. For example, a defender's roll of 4+3 becomes 5+3; 5+5 becomes 6+5, 6+1 stays 6+1, and 2 becomes 3 (the case where you're only rolling one die).

Note that this roughly flips the normal attacker's advantage - ordinarily, as long as both sides are rolling max dice, the attacker loses 0.9 units and the defender loses 1.1 on average, but with a bunker it's 1.1 and 0.9 the other way.