As quality of service gains more and more attention, bandwidth controllers gradually become one of the most important network systems used in modern Internet environment. The demand for high-performance in-line bandwidth controllers is driven by the growing bandwidth available in the last mile WAN links as well as the sophisticated packet processing functions that become essential in current computer networks. In this paper, we propose an adaptive clustering scheme to scale the throughput of in-line devices and implement the bandwidth control functions over a cluster of in-line devices. The proposed scheme aggregates the processing power of multiple in-line devices in the cluster by making incoming traffic self-dispatched in a transparent fashion, and incorporates a flow migration mechanism that keeps the load of each device balanced. The resulted cluster is also able to tolerate device failures and hence is run-time reconfigurable. Based on the proposed scheme, we successfully design a distributed policy adjustment algorithm, the proportional bandwidth allocation algorithm, and implement a clustered bandwidth controller over embedded Linux. The results of performance evaluation suggest that the proposed traffic redistribution mechanism and distributed policy adjustment algorithm can be used together to realize high-performance and reconfigurable bandwidth controllers.