Characteristics of a Standard OSPF Area

The following list provides general characteristics of an OSPF area:

• Areas contain a group of contiguous hosts and networks.
• Routers have a per-area topological database and run the same SPF algorithm.
• Each area must be connected to the backbone area known as area 0.
• Virtual links can be used to connect to area 0 in emergencies.
• Intra-area routes are used for routes within to destinations within the area.

Standard Area Design Rules

Consider the following requirements when designing an OSPF area:

• A backbone area, known as area 0, must be present.
• All areas must have a connection to the backbone area, even stub areas.
• The backbone area must be contiguous.
• Only use virtual links as an emergency temporary measure.

Area 0: The OSPF Backbone Area

A backbone area is the logical and physical structure for the AS and is attached to multiple areas. The backbone area is responsible for distributing routing information between nonbackbone areas. The backbone must be contiguous, but it need not be physically contiguous; backbone connectivity can be established and maintained through the configuration of virtual links.