In the ACI Object Model, which element restricts visibility into the ACI fabric?

The correct answer is Tenant. In the ACI (Application Centric Infrastructure) Object Model, the Tenant serves as a fundamental construct that defines the logical boundary for managing resources and policies within the ACI fabric. It essentially acts as a container for all the other objects associated with an application or service, including Endpoint Groups (EPGs), Bridge Domains (BDs), and VRFs.

The Tenant is important because it restricts visibility and access within the ACI fabric, ensuring that resources allocated to different tenants are isolated from each other. This isolation is crucial for security and operational efficiency, allowing multiple tenants to coexist on the same physical infrastructure without interfering with one another's network policies or traffic flows. Each tenant can define its own networking policies, security policies, and application requirements independently from other tenants.

In contrast, other elements such as VRF, Bridge Domain, and Endpoint Groups operate within the context of a Tenant. While they influence certain aspects of traffic handling and policy application, they do not serve the primary function of restricting visibility at the broader architectural level that the Tenant provides. Thus, the Tenant is the correct choice as it encapsulates the principles of multi-tenancy and policy enforcement within the ACI framework.