The answer is that the configuration is rejected by IOS because the backbone area cannot be a stub. OSPF area 0, the backbone, must maintain full routing visibility to interconnect all other areas, and configuring it as a stub would illegally block Type 5 LSAs and inter-area routes, violating the protocol’s fundamental design. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of OSPF area types and the backbone’s non-negotiable role; a common trap is assuming any area can be a stub, but area 0 is explicitly excluded. A reliable memory tip is “Backbone is never a stub—it must carry the full routing hub.”
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Consider this OSPF configuration on router R6:
router ospf 1
network 10.0.0.0 0.255.255.255 area 0
area 0 stub
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The configuration is rejected by IOS because the backbone area cannot be a stub.
The area 0 stub command is invalid because area 0 (the backbone) cannot be configured as a stub area. Stub areas are used to reduce LSA flooding, but the backbone must have full routing information. This configuration will be rejected by IOS.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Area 0 will become a stub area, blocking type 5 LSAs from entering the backbone.
Why it's wrong here
Incorrect. Area 0 cannot be a stub area.
✓
The configuration is rejected by IOS because the backbone area cannot be a stub.
Why this is correct
Correct. RFC 2328 prohibits the backbone from being a stub.
The command is ignored, and area 0 remains a normal area.
Why it's wrong here
Incorrect. The command is rejected, not ignored.
✗
Only type 5 LSAs are blocked, but type 3 and 4 LSAs are still allowed.
Why it's wrong here
Incorrect. The command is invalid.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
Incorrect. The command is rejected, not ignored.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The configuration is rejected by IOS because the backbone area cannot be a stub. — The area 0 stub command is invalid because area 0 (the backbone) cannot be configured as a stub area. Stub areas are used to reduce LSA flooding, but the backbone must have full routing information. This configuration will be rejected by IOS.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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