Primary issues in the design of the Hierarchical Routing Architectures include (Banerjee 2004):
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Determination of the optimal number of levels of routing hierarchies, |
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trust-mapping, |
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QoS-mapping, |
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cost-mapping, |
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translation of data between different interior and exterior routing protocols (the Border Routers face this |
| problem as they mediate between the interior and exterior routing domains) and |
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choice of parameters and mechanisms for collaborative network monitoring. |
OSPF implements a two-tier hierarchical routing model that uses a core or backbone tier known as area zero. Attached to that backbone via area border routers (ABRs) are a number of secondary tier areas. This hierarchical restriction ensures that OSPF networks scale gracefully without becoming confusing morasses of links and routers (Mark A. Sportack 1999).