In an OSPF network design, what is the primary purpose of using areas such as backbone area 0 and non-backbone areas?

Introduction / Context:
This question is about the role of areas in an OSPF (Open Shortest Path First) network. OSPF is a link-state routing protocol, and its design includes the concept of areas such as the backbone area 0 and multiple non-backbone areas. Understanding why we divide an OSPF network into areas is a key objective in Cisco certification exams.

Given Data / Assumptions:

• We are using OSPF as an interior gateway protocol inside a single autonomous system.
• The network may contain many routers and subnets, so scalability and stability are important.
• At least one backbone area (area 0) and one or more non-backbone areas can be created.

Concept / Approach:
OSPF is a link-state protocol, which means routers maintain a link-state database describing the topology of the entire area and run the SPF (Shortest Path First) algorithm to compute routes. In a single large area, every router would need to store and process LSAs for the entire network. By dividing the network into smaller areas and using area border routers (ABRs), OSPF reduces the size of each link-state database, limits the scope of LSA flooding, and contains topology changes within one area. This improves scalability and reduces CPU and memory load.

Step-by-Step Solution:
Step 1: Recognize that OSPF areas create a hierarchy: all non-backbone areas connect to area 0, which acts as the core.Step 2: Within each area, routers maintain a full link-state database and run SPF, but they do not store detailed topology information from other areas.Step 3: ABRs summarize and advertise inter-area routes so that routers learn reachability to other areas without full topology details.Step 4: Topology changes, link flaps, or LSA updates are mostly contained within the local area, limiting how far floods propagate.Step 5: Because each router only runs SPF on a reduced set of LSAs, convergence times and CPU usage are improved compared to a flat single-area design.

Verification / Alternative check:
When you compare a design with one large OSPF area to a design with multiple areas, you see that in the multi-area design, routers in a remote area do not receive every detailed LSA from other areas. Instead, they learn summarized inter-area routes, which clearly demonstrates how areas reduce database size and SPF workload.

Why Other Options Are Wrong:
Option B is incorrect because encryption and authentication in OSPF are independent of the concept of areas and are configured with additional security features.Option C is wrong because OSPF areas all use OSPF; you do not run different routing protocols in different OSPF areas.Option D is false because areas actually reduce the requirement for full mesh adjacencies; not all routers need to be neighbors.

Common Pitfalls:
A frequent misunderstanding is that areas are purely administrative or used only for policy. In reality, their key purpose is scalability and stability. Another pitfall is misconfiguring ABRs or failing to connect all non-backbone areas to area 0, which can break proper inter-area routing.

Final Answer:
The main reason to use areas is that To scale OSPF by reducing the size of each router's link-state database, limiting flooding, and minimizing SPF calculations through hierarchical design.