A site broadcasting its local value to all other sites in a fault-prone environment is a fundamental paradigm in constructing reliable distributed systems. Time complexity lower bounds and network connectivity requirements for reliable broadcast protocols in point-to-point communication networks are well known. In this paper we consider the reliable broadcast problem in distributed systems with broadcast networks (for example, Ethernets) as the basic communication architecture. We show how properties of such network architectures can be used to effectively restrict the externally visible behavior of faulty processors. We use these techniques to derive simple protocols that implement reliable broadcast in only two rounds, independent of the failure upper bounds.