Research on the network-based and distributed intrusion detection was aroused by the burst-outs of large-scale security events.How to place detection instruments was the key to the detections.The problem of detector placement was turned to that of the clustering of topology graph.A novel bidirectional hierarchical clustering algorithm was put forward

which decreased the amount of result clusters by integration of initial marker selection method based on node out-degree.The simulation results demonstrates that our clustering approaches effectively identify clusters and been employed in the measured real network of the backbone.