  The Network Function Virtualization (NFV) paradigm is enabling flexibility,
programmability and implementation of traditional network functions into
generic hardware, in form of the so-called Virtual Network Functions (VNFs).
Today, cloud service providers use Virtual Machines (VMs) for the instantiation
of VNFs in the data center (DC) networks. To instantiate multiple VNFs in a
typical scenario of Service Function Chains (SFCs), many important objectives
need to be met simultaneously, such as server load balancing, energy efficiency
and service execution time. The well-known \emph{VNF placement} problem
requires solutions that often consider \emph{migration} of virtual machines
(VMs) to meet this objectives. Ongoing efforts, for instance, are making a
strong case for migrations to minimize energy consumption, while showing that
attention needs to be paid to the Quality of Service (QoS) due to service
interruptions caused by migrations. To balance the server allocation strategies
and QoS, we propose using \emph{replications} of VNFs to reduce migrations in
DC networks. We propose a Linear Programming (LP) model to study a trade-off
between replications, which while beneficial to QoS require additional server
resources, and migrations, which while beneficial to server load management can
adversely impact the QoS. The results show that, for a given objective, the
replications can reduce the number of migrations and can also enable a better
server and data center network load balancing.
