A method for defining a look-up system having functionality suitable for a network element of a software-defined network “SDN” includes maintaining (401) data specifying a set of look-up tables so that the look-up system is capable of being constructed in alternative ways using one or more look-up tables of the set of look-up tables and each of the look-up tables has a look-up table specific resource consumption estimate indicating a load factor of at least one component of the network element when running operations related to the look-up table. The method includes selecting (402), on the basis of the look-up table specific resource consumption estimates, such one of the alternative ways of constructing the look-up system which provides a look-up system that is optimal, or at least suitable, for the network element.