Enterprise network security is typically reactive, and it relies heavily on host security and middleboxes. This approach creates complicated interactions between protocols and systems that can cause incorrect behavior and slow response to attacks. We argue that imbuing the network layer with mechanisms for dynamic access control can remedy these ills. We propose Resonance, a system for securing enterprise networks, where the network elements themselves enforce dynamic access control policies based on both flow-level information and real-time alerts. Resonance uses programmable switches to manipulate traffic at lower layers; these switches take actions (e.g., dropping or redirecting traffic) to enforce high-level security policies based on input from both higher-level security policies and distributed monitoring and inference systems. We describe the design of Resonance, apply it to Georgia Tech's network access control system, show how it can both overcome the current shortcomings and provide new security functions, describe our proposed deployment, and discuss open research questions.