  We determine a general upper bound for the steady-state entanglement
achievable by continuous feedback for systems of any number of bosonic degrees
of freedom. We apply such a bound to the specific case of parametric
interactions - the most common practical way to generate entanglement in
quantum optics - and single out optimal feedback strategies that achieve the
maximal entanglement. We also consider the case of feedback schemes entirely
restricted to local operations and compare their performance to the optimal,
generally nonlocal, schemes.
