Cisco Certified Internetwork Expert (CCIE) Practice Test

OpFlex is a system that uses a declarative policy model for distributed control. This means that it allows users to specify what state they want the network to be in, rather than detailing the steps needed to reach that state. In OpFlex, policies are defined in such a way that they can be implemented across various devices in a distributed manner, allowing for greater flexibility and scalability in network management.

This approach enables simpler policy enforcement and reduces the complexity associated with configuration management, as the underlying infrastructure can interpret and implement these high-level policy statements. Consequently, it aligns well with concepts in Software-Defined Networking (SDN), where control is abstracted away from the devices and centralized in a policy control plane.

The other options, while related to networking and certain aspects of control systems, do not specifically employ a declarative policy model in the same way. OpenFlow primarily defines a protocol for the communication between a controller and switches but does not inherently focus on declarative policies. NSX is more about network virtualization features and security, while VXLAN deals with extending Layer 2 networks over Layer 3 infrastructure and does not utilize a declarative model.