Flooding Domains versus Areas
At a fundamental level, SPF and IS-IS are similar in operation. They both build neighbor adjacencies. They both use Dijkstra’s shortest path first (SPF) to find the shortest path to every destination in the network. They both advertise the state of each link connected to a network device. There are some differences, of course, such as the naming (OSI addresses versus IP addresses, intermediate systems versus routers). Many of the similarities and differences don’t play too much in the design of a network, though.
One difference that does play into network design, however, is the way in which the two protocols break up a single failure domain into multiple failure domains. In OSPF we have areas, while in IS-IS we have flooding domains. What’s the difference between these two, and how does it effect network design? Let’s use the illustration below as a helpful reference point for the two different solutions.
In the upper network, we have an illustration of how OSPF areas work. Each router at the border of a flooding domain (an Area Border Router, or ABR), has a certain number of interfaces in each area. Another way of saying this is that an OSPF ABR is Continue reading
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