TCP/IP Tutorial and Technical Overview - IBM Redbooks

Subdividing the network provides the following benefits:
► Within an area, every router maintains an identical topology database
describing the routing devices and links within the area. These routers have
no knowledge of topologies outside the area. They are only aware of routes to
these external destinations. This reduces the size of the topology database
maintained by each router.
► Areas limit the potentially explosive growth in the number of link state
updates. Most LSAs are distributed only within an area.
► Areas reduce the CPU processing required to maintain the topology
database. The SPF algorithm is limited to managing changes within the area.
Backbone area and area 0
All OSPF networks contain at least one area. This area is known as area 0 or the
backbone area. Additional areas can be created based on network topology or
other design requirements.
In networks containing multiple areas, the backbone physically connects to all
other areas. OSPF expects all areas to announce routing information directly into
the backbone. The backbone then announces this information into other areas.
Figure 5-14 on page 198 depicts a network with a backbone area and four
additional areas.
Chapter 5. Routing protocols 197