Monitoring inter-domain route timely and dynamically is a hard issue. The paper introduces a BGP passive model, and analyzes the possible BGP network events above the established model, proposes a novel approach to monitor BGP network and analyzes BGP dynamics based on the BGP routing interaction. A monitor agent (MA) partially implementing BGP protocol engine is introduced, so that it can listen and receive BGP routing update message by peering with any BGP speaker in the managed BGP network. Based on the collected routing information, the MA can re-construct the BGP topology, and dynamically reflect the topology change triggered by the incoming routing update message, and analyze the BGP network route flap with the inter-arrival time. Comparing with conventional approaches, the proposed approach has better feature in timeliness and dynamicity, and lower overhead incurring in both network and routers.