Network Functions Virtualization

Implementing OpenFlow protocol is the best solution to improve the communication between the SDN controller and the VNFs. It is because OpenFlow is a well-known communictaion protocol in SDN environments used to deliver instructions from the SDN controller to the switches, including vSwitches in a VNF context. This can increase the efficiency and reduce latency. Adding more physical switches or upgrading to a faster NIC could lead to improvements in certain situations, but they don't precisely address the specified communication issue between SDN controller and VNFs. On the other hand, replacing the SDN controller with a hardware-based switch is not compatible with the NFV infrastructure.

The most important characteristic for an enterprise to prioritize in this context is 'Interoperability'. Network Functions Virtualization (NFV) involves managing and deploying network services through software, rather than hardware. To save on expenditure and operational costs, it's vital that these software-based network functions can communicate and work seamlessly with each other, regardless of the vendor. This is achieved through interoperability.

While 'High Scalability', 'Centralized Management', and 'Minimal Redundancy' are also important characteristics of NFV, they do not directly address the enterprise's need for an easy integration of different network function vendors into their existing infrastructure.

• 'High Scalability' helps to expand the network functions to meet growing demand, but doesn't ensure smooth integration of different vendor's products.
• 'Centralized Management' provides easier control and coordination of network functions, but doesn't ensure other vendors' network functions can integrate easily.
• 'Minimal Redundancy' can contribute to saving costs, but it also doesn't guarantee smooth incorporation of various vendors' network functions.

The Virtual Network Function Manager (VNFM) is the correct answer as it is primarily responsible for the lifecycle management of Virtual Network Functions (VNFs). The VNFM handles areas such as instantiation, scaling, healing and termination of VNFs within an NFV infrastructure. The other options are not solid choices because the Virtualized Infrastructure Manager (VIM) manages the virtualization layer and the virtual resources but it doesn't handle the lifecycle management of VNFs. The Network Functions Virtualization Orchestrator (NFVO) on the other hand, oversees the execution of various network services and also overlooks lifecycle management but it is not directly involved with the lifecycle management of individual VNFs. The Software-Defined Networking Controller (SDN) involves in routing and switching but it isn't involved with lifecycle management of VNFs.