In OSPF, why are intra-area summary routes not allowed, and where is summarization designed to occur instead?

Introduction / Context:
This question probes your understanding of how OSPF maintains a consistent view of the network within each area and where route summarization fits into that design. OSPF is a link-state protocol, and intra-area behavior is tightly tied to the requirement that all routers in the same area share an identical link-state database for that area.

Given Data / Assumptions:
- OSPF uses link-state advertisements to build a complete picture of the area topology.
- All routers in a given area must calculate routes using the same information.
- Summarization groups multiple specific routes into a single, more general route.
- The question asks why summarization is not performed inside an area and where it is normally done instead.

Concept / Approach:
Within an OSPF area, routers flood LSAs describing every link and network in that area. Each router runs the Shortest Path First algorithm on the full intra-area topology. If some routers summarized intra-area routes and others did not, the topology views would diverge, breaking the fundamental OSPF requirement of identical LSDBs. To preserve consistency, OSPF performs summarization at area boundaries or at redistribution points, where abstraction of internal detail is appropriate and does not compromise SPF calculations inside the area.

Step-by-Step Solution:
Step 1: Recall that OSPF is a link-state protocol, where every router in the same area must share an identical link-state database for that area.Step 2: Understand that intra-area routes are calculated directly from this LSDB using the same SPF algorithm on every router.Step 3: Realize that if intra-area summaries were allowed, some routers might see only summarized routes while others saw full specifics, leading to inconsistent shortest path calculations.Step 4: Recognize that OSPF instead allows summarization at Area Border Routers between areas (for example, using area range commands) and at Autonomous System Boundary Routers summarizing external routes.Step 5: Conclude that intra-area summarization is prohibited to maintain LSDB consistency, while inter-area and external summarization is explicitly supported.

Verification / Alternative check:
In configuration guides, you see commands such as area 1 range or summary-address applied on ABRs and ASBRs, not on arbitrary routers inside an area. When you examine LSDBs within a single area, you find detailed LSAs describing all networks, not summarized aggregates. Only when you move between areas or from OSPF into another routing domain do you typically see summarization applied, confirming that OSPF intentionally keeps full detail within each area.

Why Other Options Are Wrong:
Option A is incorrect because OSPF is a link-state protocol, not purely distance-vector, and it does support summarization in the right places. Option C limits summarization to special area types, which is not true; summarization is an ABR or ASBR function across all appropriate areas. Option D is wrong because OSPF explicitly supports VLSM and CIDR. Option E incorrectly ties summarization to DR elections, which are separate mechanisms used on multi-access networks.

Common Pitfalls:
A frequent misunderstanding is to assume that summarization can be applied anywhere, as in some distance-vector protocols. In OSPF, the need for identical LSDBs within an area restricts where summarization can occur. Another pitfall is not recognizing the distinct roles of ABRs and ASBRs in controlling the level of detail shared between areas or between OSPF and external domains. Remembering that “inside an area = full detail, at the edge = summarization” is a useful rule of thumb.

Final Answer:
Intra-area summary routes are not allowed in OSPF because all routers in an area must share a complete and consistent link-state database, so OSPF performs summarization only at ABRs or ASBRs toward other areas or external domains.