In this paper we propose a passive conformance testing technique applied to a Mobile ad hoc network (MANET) routing protocol, OLSR, that is characterized by a dynamically changing topology and lack of centralized management. This makes it necessary to investigate new ways to test complex scenarios and configurations. The work here proposes a formal passive testing method to test the conformance and reliability of the protocol. The method developed has been performed on a real case study showing that the approach can be successful applied and that it allows reducing inconclusive verdicts often observed using other methods.