Checking whether a network correctly implements intended policies is challenging even for basic reachability policies (Can X talk to Y?) in simple stateless networks with L2/L3 devices. In practice, operators implement more complex context-dependent policies by composing stateful network functions; e.g., if the IDS flags X for sending too many failed connections, then subsequent packets from X must be sent to a deep-packet inspection device. Unfortunately, existing approaches in network verification have fundamental expressiveness and scalability challenges in handling such scenarios. To bridge this gap, we present BUZZ, a practical model-based testing framework. BUZZ’s design makes two key contributions: (1) Expressive and scalable models of the data plane, using a novel high-level traffic unit abstraction and by modeling complex network functions as an ensemble of finite-state machines; and (2) A scalable application of symbolic execution to tackle state-space explosion. We show that BUZZ generates test cases for a network with hundreds of network functions within two minutes (five orders of magnitude faster than alternative designs). We also show that BUZZ uncovers a range of both new and known policy violations in SDN/NFV systems.