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Network functions virtualization is an architecture of network services -- usually Layer 4 through 7 -- deployed as software, often as virtual machines, and separated from the underlying hardware. Virtualized network functions are the implementation of the network services. This process enables flexible deployment and orchestration of network functions.
Given the benefits of network functions virtualization, or NFV, enterprises might ask which core network functions are being virtualized. The answer is most commonly used network functions are undergoing virtualization. These include popular ones, such as routing, security -- firewalls, for example -- WAN optimization, network address translation, domain name system, Dynamic Host Configuration Protocol and IP address management.
Many of these functions were traditionally delivered as dedicated middleboxes, and these devices can represent a significant part of an enterprise's network architecture. Now, these middleboxes can be deployed as virtual network functions (VNFs).
Branch office and data center network functions
The use of network functions virtualization is especially attractive within branch offices, as core network functions can be easily deployed and managed, which reduces the need for maintenance visits at branch offices.
Multiple functions may be consolidated into devices provided by a network vendor, and third-party VNFs may be installed alongside those provided by network vendors. Additionally, these VNFs can be chained together to create complete network processes.
Branch routers are being virtualized, as are routers in large data centers. Even telecom carriers use NFV as part of their infrastructure. Practically speaking, however, some routing options with high-performance requirements may need purpose-built hardware instead of VNFs.
Virtual switches aren't VNFs
Are there cases when virtualizing network functions isn't possible? Other than performance -- when the issue of sufficient network performance might guide users to avoid virtualization -- they all can be virtualized. But a few special cases need clarification.
Switches -- such as those found at the top of server racks, for example -- are physical Layer 2 or 3 network devices. These are not deployed as VNFs, because switching functions need hardware-based network processors to forward packets for high performance and low latency. Further, switching isn't considered a network function in the same sense as a service provided by Layer 4 through 7 middleboxes. Some switches may host VNFs as an added feature, but the core switching functions are not virtualized.
One case regarding virtual switches may cause confusion: Hypervisor platforms can provide built-in virtual switches that enable virtual machines to communicate, but virtual switches are usually not considered VNFs. Therefore, hypervisor switches can be virtual, but they aren't considered virtual network functions, like the Layer 4 to 7 services described earlier. It's confusing, but it's the way the layers of the technologies are defined.
In summary, most network functions -- in particular, those providing middlebox capabilities -- support NFV. The VNFs can run on premises within data centers, branches and, in some cases, cloud providers. Enterprises should have little hesitation when considering whether to virtualize network functions.