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Network Function Virtualization (NFV) and Service Function Chaining (SFC) have been widely used to enable flexible and agile network management. To enhance reliability, some research has proposed to deploy backup function instances for prompt recovery when a primary instance fails. While most of the recent studies focus on speeding up recovery, less attention has been paid to the problem of minimizing the state update cost. In this work, we present PiggyBackup (Piggyback-based Backup), an efficient backup instance deployment and update protocol. Our key idea is to reuse the existing service chains traversing through servers in a network to help piggyback the update information. By doing this, we eliminate the header overhead and reduce the amount of update traffic significantly. To realize such a piggyback-based update more efficiently, we investigate the backup instance deployment and chain selection problems to enhance piggybacking opportunities and reduce the forwarding hop counts with explicit consideration of the distribution of service chains. Our simulation results show that PiggyBackup reduces the average overall update overhead by 47.65% and 39.56%, respectively, in a fat-tree topology as compared to random deployment and shortest path based deployment.