This paper analyzes both the strong points and weak points of OSPF routing protocol from security perspective. On its strong points, we abstract its features of information least dependency and information hiding, which make it very robust and fault resilient, even when facing certain malicious attacks. On its weak points, we take a pragmatic look at various problems centering round secure routing protocols. By carefully investigating a special re-routing attacking case, we show how an home-made malicious router can easily disrupt the service. It also provides a concrete example for routing protection and intrusion detection. Finally, we present the active protection idea and its architectural flexibility and compatibility advantages.