The answer is the OSPF network type mismatch between point-to-point and broadcast causes R1 to reject the type-3 LSA because the LSA contains a DR field that is invalid for point-to-point links. When R1 is configured with `ip ospf network point-to-point`, it expects no DR/BDR election and uses a different LSA format that omits the DR field, while R2, operating with the default broadcast type, includes a DR in its LSAs. Although the adjacency forms in FULL state on both sides—a common trap—the mismatch prevents R1 from installing the inter-area route because it cannot process the LSA’s DR field. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that OSPF neighbor state does not guarantee correct LSA propagation; the network type must match for proper route installation. Memory tip: “Full neighbors, empty table—check the DR field on point-to-point.”
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
Router R1 and R2 are connected via a serial link running OSPF. R1 has 'ip ospf network point-to-point' configured on the interface, while R2 has default broadcast network type. R1's 'show ip ospf neighbor' shows R2 in FULL state, but R2's 'show ip ospf neighbor' shows R1 in FULL state. However, R1's routing table does not contain a route to a subnet 10.10.10.0/24 that is advertised by R2 via a type-3 LSA. R2's 'show ip route 10.10.10.0' shows it as connected. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The network type mismatch causes R1 to not install the type-3 LSA because the LSA has a DR field that is not valid for point-to-point networks.
Network type mismatch on a point-to-point link can cause OSPF adjacency to form but may affect LSA propagation. With point-to-point, R1 expects no DR/BDR election and uses a different LSA format. R2, with broadcast, expects DR/BDR. The adjacency forms because both see each other as neighbors, but the type-3 LSA from R2 might be generated incorrectly or R1 may not accept it due to network type mismatch. The correct answer is that the network type mismatch causes R1 to ignore type-3 LSAs from R2 because R1 expects the link to be point-to-point and R2's LSA includes a DR field that R1 does not process.
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.
✓
The network type mismatch causes R1 to not install the type-3 LSA because the LSA has a DR field that is not valid for point-to-point networks.
Why this is correct
OSPF network type mismatch can lead to LSA filtering; point-to-point interfaces ignore LSAs with DR information.
R2's OSPF process has 'distance 150' set, making the route unreachable.
Why it's wrong here
AD does not affect LSA installation; it affects route preference.
✗
R1 has a distribute-list blocking the 10.10.10.0/24 route.
Why it's wrong here
No distribute-list mentioned.
✗
The subnet 10.10.10.0/24 is not in R2's OSPF database due to a missing network command.
Why it's wrong here
R2 shows it as connected, but OSPF must be enabled on that interface.
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
R2 shows it as connected, but OSPF must be enabled on that interface.
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.
Administrative Distance — This question tests Administrative Distance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The network type mismatch causes R1 to not install the type-3 LSA because the LSA has a DR field that is not valid for point-to-point networks. — Network type mismatch on a point-to-point link can cause OSPF adjacency to form but may affect LSA propagation. With point-to-point, R1 expects no DR/BDR election and uses a different LSA format. R2, with broadcast, expects DR/BDR. The adjacency forms because both see each other as neighbors, but the type-3 LSA from R2 might be generated incorrectly or R1 may not accept it due to network type mismatch. The correct answer is that the network type mismatch causes R1 to ignore type-3 LSAs from R2 because R1 expects the link to be point-to-point and R2's LSA includes a DR field that R1 does not process.
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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