The Network Function Virtualization (NFV) paradigm is rapidly gaining interest among Internet Service Providers (ISPs). However, the transition to this paradigm on ISP networks comes with a unique set of challenges, namely legacy equipment already in-place, heterogeneous traffic from multiple clients, and very large scalability requirements. In this paper we thoroughly analyze such challenges and discuss NFV design guidelines that address them efficiently. Particularly, we show that a

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... zation of the NFV control while maintaining global state improves scalability, offers better per-flow decisions and simplifies the implementation of Virtual Network Functions (VNFs). Building on top of such principles, we propose a partially decentralized NFV architecture enabled via an enhanced Software Defined Networking (SDN) infrastructure. We also perform a qualitative analysis of the architecture to identify advantages and challenges. Finally, we determine the bottleneck component, based on the qualitative analysis, which we implement and benchmark in order to assess the feasibility of the architecture.