In this paper, we focus on a problem that directly affects the efficiency of IP packet forwarding: routing instability. Different with most of the other research which studies inter-domain routing instability, we analyze routing instability from intra-domain aspect because we believe it is the oscillation of intra-domain routing information that directly causes instability of inter-domain routing. We show the statistics result of OSPF routing information that we collected on WIDE Internet during 9-month period. From our analysis, we find that although network operators generally don't notice, serious route flaps occur frequently. We divide the oscillation of OSPF routing information into several categories according to its characteristic. Basing on the statistics results, we summarize the causes of observed oscillations as network congestion, misconfiguration and the bug of routing software. We also suggest the use of more flexible parameter of routing protocol to improve network reachability while preserving normal routing stability. Although we chose OSPF as our target routing protocol in this research, the result is considered to be applicable to other link state intra-domain routing protocols such as IS-IS.