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OSPF Routing Protocol: Popular and Robust
Traffic Cop For Your Routing Domain
October 22, 2008
OSPF is the most widely used IGP. When we discuss IGPs, we�re talking about one routing domain, or Autonomous System (AS). Imagine a medium-sized company with multiple buildings and departments, all connected together and sharing two redundant Internet links. All of the buildings on-site are part of the same AS. But with OSPF we also have the concept of an Area, which allows further segmentation, perhaps by department in each building.
To understand the design needs for areas in OSPF, let�s start by discussing how OSPF works. There�s some terminology you may not have encountered before, including:
First, when a router running OSPF comes up it will send hello packets to discover its neighbors and elect a designated router. The hello packet includes link-state information, as well as a list of neighbors. Providing information about your neighbor to that neighbor serves as an ACK, and proves that communication is bi-directional. OSPF is smart about the layer 2 topology: if you�re on a point-to-point link, it knows that this is enough, and the link is considered �up.� If you�re on a broadcast link, the router must wait for an election before deciding if the link is operational.