Contrary to common wisdom, OSPF is not a pure link-state protocol. It uses link state algorithms within an area, but behaves almost like a distance vector protocol between the areas. This distinction introduces temporary routing instabilities into multi-area OSPF network that does not use inter-area summarization. In today's IP Corner article, Ivan Pepelnjak illustrates how this unexpected behavior can affect the convergence of your network and how you can use proprietary extensions of Cisco IOS to alleviate the undesired side effects of OSPF.