OSPF works best in a hierarchical routing environment. When designing an OSPF network, the first and most important task is to determine which routers and links are to be included in the backbone (area 0) and which are to be included in each area. The following are three important characteristics to OSPF to ensure that your OSPF network has a hierarchical routing structure:
• The hierarchical routing structure must exist or be created to effectively use OSPF. The benefits of having a single area include simplicity, ease of troubleshooting, and so on.
• A contiguous backbone area must be present, and all areas must have a connection to the backbone.
• Explicit topology (shortest path) has precedence over any IP addressing schemes that might have been applied; that is, your physical topology takes precedence over a summarized route.
When designing the topology for an OSPF network, consider the following important items:
• Number of routers in an area
• Number of areas connected to an ABR
• Number of neighbors for a router
• Number of areas supported by a router
• Selection of the designated router (DR)
• Size and development of the OSPF LSDB These topics are covered in the following sections.
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