Because of today's increased traffic volume and sophisticated attacks, implementing a network intrusion detection/prevention system (NIDS/NIPS) with a single workstation has been challenging. In this paper, we propose Brownie, a system for improving performance by means of collaboration between already-existing NIDSs, instead of installing one expensive hardware or parallel NIDS at a network entry point. Our Brownie achieves performance improvement by 1) offloading overloaded NIDS, and 2) eliminating redundant rules. First, a Brownie exchanges NIDSs' load status and transfers some rules from overloaded to light-loaded NIDSs, which prevents the overloaded NIDSs from bottlenecking the network. Second, if some NIDSs in a network path enable the same rules, a Brownie eliminates the redundant rules, which reduces the aggregate overhead of the NIDSs. The experimental results with a university full-packet trace suggest that Brownies successfully offloads overloaded NIDS and eliminates redundant rules.