By default, what is the key difference between the types of routes allowed into IS-IS areas and OSPF areas in a multiarea design?

Introduction / Context:
This question compares Intermediate System to Intermediate System and OSPF in terms of how they handle routing information across different areas or levels. Both are link-state protocols, but they organize their topologies differently. Understanding this difference is important when reading diagrams, interpreting exam questions, and troubleshooting multiarea deployments in service provider and large enterprise environments.

Given Data / Assumptions:
- We are dealing with IS-IS and OSPF multiarea or multilevel designs.
- IS-IS uses Level 1 and Level 2 routers and areas, whereas OSPF uses numbered areas with ABRs and LSAs.
- The question asks about the types of routes that are allowed into these areas or levels by default.
- We focus on the high-level behavior rather than every special area type or tuning option.

Concept / Approach:
In IS-IS, Level 1 routers know only about their own area. They maintain routes to destinations within their area and rely on a default route pointing to the nearest Level 2 router for destinations outside the area. Level 2 routers form a backbone that knows about inter-area reachability. In OSPF, routers inside a normal area maintain detailed intra-area routes and receive inter-area summary routes from ABRs (Type 3 LSAs), as well as external routes (Type 5 or Type 7 LSAs) depending on area type. Therefore, OSPF normal areas can receive specific inter-area and external routes, while IS-IS Level 1 areas rely heavily on a default route to reach destinations beyond their area.

Step-by-Step Solution:
Step 1: Recall that IS-IS distinguishes between Level 1 (intra-area) and Level 2 (backbone) routing.Step 2: Understand that Level 1 routers know detailed routes only inside their own area and get a default route to a Level 2 router for everything else.Step 3: Recognize that Level 2 routers know how to reach all areas and exchange inter-area information.Step 4: Contrast this with OSPF, where routers in a normal area have intra-area LSAs and also receive explicit inter-area summary routes and, unless the area is stubby, external LSAs.Step 5: Choose the option that accurately captures this difference in the default behavior of IS-IS Level 1 areas versus OSPF normal areas.

Verification / Alternative check:
Design examples in service provider networks often use IS-IS with Level 1-2 routers at the edge. Customer or access areas contain Level 1 routers that have detailed routing only within their area, plus a default route toward the core. In contrast, enterprise OSPF designs typically have branch areas receiving not only a default route but also specific inter-area routes and sometimes external prefixes. Examining sample configurations and route tables from lab topologies helps confirm these theoretical differences.

Why Other Options Are Wrong:
Option A incorrectly claims that OSPF areas allow only intra-area routes, which is untrue; OSPF normal areas receive inter-area and possibly external LSAs. Option B exaggerates, stating that OSPF areas always have only a default and IS-IS always has full Internet tables, which is not a default behavior. Option D falsely claims that IS-IS does not support multiarea designs and that OSPF uses a flat structure, which is completely incorrect. Option E ignores the well-documented structural differences between the protocols and is therefore wrong.

Common Pitfalls:
Students sometimes assume that because both protocols are link-state, they must behave identically at the area or level boundaries. Another pitfall is confusing the role of a Level 1-2 router in IS-IS with an OSPF ABR without understanding how default routes are used in IS-IS Level 1 areas. Keeping the mental model clear—IS-IS uses levels and defaults at Level 1, OSPF uses areas with explicit summaries and externals—helps avoid these misunderstandings.

Final Answer:
By default, IS-IS Level 1 routers learn only intra-area routes plus a default route pointed at the nearest Level 2 router, while OSPF routers in a normal area learn detailed intra-area, inter-area and external routes based on the LSA types flooded into that area.