  Optimal placement of Virtual Network Functions (VNFs) in virtualized data
centers enhances the overall performance of Service Function Chains (SFCs) and
decreases the operational costs for mobile network operators. Maintaining an
optimal placement of VNFs under changing load requires a dynamic
reconfiguration that includes adding or removing VNF instances, changing the
resource allocation of VNFs, and re-routing corresponding service flows.
However, such reconfiguration may lead to notable service disruptions and
impose additional overhead on the VNF infrastructure, especially when
reconfiguration entails state or VNF migration. On the other hand, not changing
the existing placement may lead to high operational costs. In this paper, we
investigate the trade-off between the reconfiguration of SFCs and the
optimality of the resulting placement and service flow (re)routing. We model
different reconfiguration costs related to the migration of stateful VNFs and
solve a joint optimization problem that aims to minimize both the total cost of
the VNF placement and the reconfiguration cost necessary for repairing a
suboptimal placement. Numerical results show that a small number of
reconfiguration operations can significantly reduce the operational cost of the
VNF infrastructure; however, too much reconfiguration may not pay off should
heavy costs be involved.
