- A
R1's point-to-point configuration disables DR/BDR election, but R2 still expects it, causing R1 to ignore R2's LSUs.
With mismatched network types, R1 sends packets without a DR, but R2 expects them from the DR, leading to packet loss.
- B
R2's DR election is incorrect because it has a higher router ID, but R1 should be the DR.
Why wrong: The issue is network type mismatch, not router ID priority.
- C
R1's interface has an MTU mismatch that causes OSPF packets to be dropped.
Why wrong: MTU mismatch would cause adjacency failure, not intermittent connectivity.
- D
R2 has a firewall that blocks OSPF packets from R1.
Why wrong: No firewall is mentioned.
Question 1,411 of 2,152
Troubleshooting OSPF Network Type Mismatch
This 300-410 practice question tests your understanding of eigrp troubleshooting. 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.
An OSPF network has a multi-access link between routers R1 and R2. R1 is configured with 'ip ospf network point-to-point' on the interface, while R2 uses the default broadcast network type. R1 shows: 'show ip ospf neighbor' lists R2 as FULL/DR, but R2 shows: 'show ip ospf neighbor' lists R1 as FULL/DROTHER. Traffic from R1 to R2's loopback is fine, but from R2 to R1's loopback is intermittent. What is the root cause?
Quick Answer
The answer is an OSPF network type mismatch, specifically that R1’s point-to-point configuration disables DR/BDR election while R2’s default broadcast mode still expects it, causing R1 to ignore R2’s LSUs. This happens because R1 treats the link as a point-to-point adjacency, so it does not participate in DR/BDR election and will not accept a DR’s LSUs from R2, which sees itself as the DR. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF network types affect neighbor state and next-hop forwarding, often appearing as a tricky question where both sides show FULL but with mismatched DR roles. A common trap is assuming FULL means full convergence, but the intermittent traffic from R2 to R1’s loopback reveals the root cause: R1 lacks the correct next-hop for R2’s routes. Remember the tip: “Point-to-point ignores the DR—if roles don’t match, LSUs won’t catch.”
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R1's point-to-point configuration disables DR/BDR election, but R2 still expects it, causing R1 to ignore R2's LSUs.
R1's 'ip ospf network point-to-point' command disables DR/BDR election on its interface, meaning R1 will not participate in the election and will not form a full adjacency with a DR or BDR. However, R2, using the default broadcast network type, expects a DR/BDR election and elects itself as DR (likely due to higher router ID). R1 sees R2 as FULL/DR because R1 still receives Hellos and forms a neighbor relationship, but R1 ignores LSUs from R2 because R1's point-to-point logic does not recognize the DR as a valid next hop for flooding, causing intermittent reachability to R1's loopback.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
R1's point-to-point configuration disables DR/BDR election, but R2 still expects it, causing R1 to ignore R2's LSUs.
Why this is correct
With mismatched network types, R1 sends packets without a DR, but R2 expects them from the DR, leading to packet loss.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
R2's DR election is incorrect because it has a higher router ID, but R1 should be the DR.
Why it's wrong here
The issue is network type mismatch, not router ID priority.
- ✗
R1's interface has an MTU mismatch that causes OSPF packets to be dropped.
Why it's wrong here
MTU mismatch would cause adjacency failure, not intermittent connectivity.
- ✗
R2 has a firewall that blocks OSPF packets from R1.
Why it's wrong here
No firewall is mentioned.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that 'ip ospf network point-to-point' only affects hello/dead timers or neighbor state, when in reality it fundamentally changes the OSPF operational model on that link, including DR/BDR election and flooding behavior.
Detailed technical explanation
How to think about this question
In OSPF broadcast networks, the DR is responsible for flooding LSUs to all other routers (using 224.0.0.6). When R1 is configured as point-to-point, it expects to send LSUs directly to R2's unicast IP (using the neighbor command or via the point-to-point link), but R2, acting as DR, sends LSUs to the multicast address 224.0.0.6, which R1 may not process correctly because R1's interface is no longer listening for OSPF multicast traffic on that segment. This mismatch causes R1 to miss some LSUs from R2, leading to intermittent routing table updates and traffic drops.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
What to study next
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FAQ
Questions learners often ask
What does this 300-410 question test?
EIGRP Troubleshooting — This question tests EIGRP Troubleshooting — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: R1's point-to-point configuration disables DR/BDR election, but R2 still expects it, causing R1 to ignore R2's LSUs. — R1's 'ip ospf network point-to-point' command disables DR/BDR election on its interface, meaning R1 will not participate in the election and will not form a full adjacency with a DR or BDR. However, R2, using the default broadcast network type, expects a DR/BDR election and elects itself as DR (likely due to higher router ID). R1 sees R2 as FULL/DR because R1 still receives Hellos and forms a neighbor relationship, but R1 ignores LSUs from R2 because R1's point-to-point logic does not recognize the DR as a valid next hop for flooding, causing intermittent reachability to R1's loopback.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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