2:1 cavalry bonus?

[unmerged(30110)]

Does the bonus refer to 2:1 cavalry to cavalry ratio or 2:1 cavalry versus infantry ratio?
If I don't have cavalry in an army, does it always suffer from this bonus when attacked by armies with cavalry?

 

[unmerged(25053)]

2 to 1 cavalry to cavalry. If the enemy does not have cavalry, then you would also get the bonus. (100% to 0).