What component is essential for Cisco's Application Centric Infrastructure (ACI)?

The essential component for Cisco's Application Centric Infrastructure (ACI) is policy enforcement. ACI is designed around a policy-driven approach to network management, which aims to automate and streamline operations within data centers. This allows for consistent application deployment and lifecycle management by defining policies that dictate how applications connect, communicate, and interact with the underlying infrastructure.

Policy enforcement in ACI enables one to define and apply networking policies that govern security, segmentation, and quality of service, aligning network resources with application requirements. This is crucial since ACI provides a flexible and scalable framework that can adjust to the dynamic needs of applications, rather than being bound to hardware configurations or static rules.

In contrast, while vertical scaling can refer to increasing resources of an individual server, it doesn't specifically relate to the infrastructure management principles of ACI. Segmentation is certainly a component but doesn't encompass the broader policy management capabilities that are integral to ACI’s architecture. Virtual switches are relevant in a virtualization context but do not capture the core policy enforcement mechanism that ACI employs to manage application requirements effectively. Thus, policy enforcement stands out as the vital component that empowers Cisco ACI to deliver its fundamental networking solutions.