We present a fuzzy logic based methodology of decision making on bandwidth allocation in a substrate network with DaVinci architecture, according to which the physical substrate network is divided into virtual networks. This methodology describes a fuzzy modification of the adaptive bandwidth allocation mechanism introduced in order to optimize decision making under uncertain network conditions by using fuzzification and defuzzification principles and the expert knowledge database of fuzzy rules. The effectiveness of it is evaluated on the link level for two traffic types within simulation experiments realized by using Coloured Petri Nets Tools.