- A
One router has 'ip ospf network point-to-point' configured, while the other uses the default broadcast, causing a mismatch in the network type.
A network type mismatch prevents proper OSPF operation; broadcast expects DR/BDR election, while point-to-point does not, leading to inconsistent neighbor states and potential forwarding issues.
- B
The FastEthernet interface is operating at half-duplex, causing packet loss.
Why wrong: Duplex mismatch could cause issues, but the OSPF neighbor state would not show FULL/DR if that were the case.
- C
OSPF hello and dead timers are mismatched, preventing adjacency formation.
Why wrong: The neighbor state is FULL, so timers are consistent.
- D
The MTU on the link is mismatched, causing OSPF packets to be dropped.
Why wrong: MTU mismatch typically prevents adjacency from forming, but here it is FULL.
Troubleshooting OSPF Network Type Mismatch: DR/BDR vs Point-to-Point
This 300-410 practice question tests your understanding of device management. 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.
Two routers R1 and R2 are connected via a FastEthernet link. OSPF is configured on both with network type broadcast. R1 shows: 'show ip ospf neighbor' lists R2 as FULL/DR. R2 shows: 'show ip ospf neighbor' lists R1 as FULL/BDR. However, pings between loopback interfaces on R1 and R2 fail intermittently. 'show ip route' on R1 shows the loopback of R2 as reachable via OSPF, but 'show ip ospf interface' on R1 shows the link as point-to-point. What is the root cause?
Quick Answer
The answer is an OSPF network type mismatch, where one router has the interface manually configured as point-to-point while the other retains the default broadcast setting. This is correct because the `show ip ospf interface` output on R1 indicating point-to-point directly contradicts the DR/BDR election states seen in the neighbor table—broadcast requires a DR/BDR, but point-to-point does not. Although the adjacency reaches FULL, the mismatch corrupts the forwarding database, causing intermittent ping failures despite routes appearing in the table. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to correlate `show ip ospf interface` and `show ip ospf neighbor` outputs to spot configuration inconsistencies, a common trap where candidates assume FULL adjacency guarantees correct traffic flow. Remember the memory tip: “If the interface type says point-to-point but the neighbor table shows a DR, you’ve got a mismatch that will break your flow.”
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
One router has 'ip ospf network point-to-point' configured, while the other uses the default broadcast, causing a mismatch in the network type.
The root cause is a mismatch in OSPF network type between R1 and R2. R1's 'show ip ospf interface' shows the link as point-to-point, indicating that 'ip ospf network point-to-point' is configured on R1, while R2 uses the default broadcast type. This mismatch causes R1 to not send OSPF hello packets with the correct multicast address (224.0.0.5 for point-to-point vs. 224.0.0.6 for broadcast DR/BDR election), leading to intermittent connectivity and the observed neighbor states (FULL/DR and FULL/BDR) that appear correct but are actually unstable.
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.
- ✓
One router has 'ip ospf network point-to-point' configured, while the other uses the default broadcast, causing a mismatch in the network type.
Why this is correct
A network type mismatch prevents proper OSPF operation; broadcast expects DR/BDR election, while point-to-point does not, leading to inconsistent neighbor states and potential forwarding issues.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The FastEthernet interface is operating at half-duplex, causing packet loss.
Why it's wrong here
Duplex mismatch could cause issues, but the OSPF neighbor state would not show FULL/DR if that were the case.
- ✗
OSPF hello and dead timers are mismatched, preventing adjacency formation.
Why it's wrong here
The neighbor state is FULL, so timers are consistent.
- ✗
The MTU on the link is mismatched, causing OSPF packets to be dropped.
Why it's wrong here
MTU mismatch typically prevents adjacency from forming, but here it is FULL.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a FULL neighbor state guarantees correct OSPF operation, but here the mismatch in network type causes subtle forwarding issues even though the adjacency appears established.
Trap categories for this question
Command / output trap
Duplex mismatch could cause issues, but the OSPF neighbor state would not show FULL/DR if that were the case.
Detailed technical explanation
How to think about this question
OSPF network type determines how hello packets are sent (unicast, multicast, or broadcast) and whether DR/BDR election occurs. When one side is configured as point-to-point, it expects to form a neighbor with a single router using 224.0.0.5, while the broadcast side expects DR/BDR election using 224.0.0.6. This mismatch can still allow a FULL adjacency to form because the broadcast router may accept the point-to-point hello, but the point-to-point router ignores DR/BDR advertisements, leading to inconsistent forwarding and intermittent reachability. In real-world scenarios, this is a common misconfiguration when migrating from broadcast to point-to-point without coordinating both ends.
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?
Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: One router has 'ip ospf network point-to-point' configured, while the other uses the default broadcast, causing a mismatch in the network type. — The root cause is a mismatch in OSPF network type between R1 and R2. R1's 'show ip ospf interface' shows the link as point-to-point, indicating that 'ip ospf network point-to-point' is configured on R1, while R2 uses the default broadcast type. This mismatch causes R1 to not send OSPF hello packets with the correct multicast address (224.0.0.5 for point-to-point vs. 224.0.0.6 for broadcast DR/BDR election), leading to intermittent connectivity and the observed neighbor states (FULL/DR and FULL/BDR) that appear correct but are actually unstable.
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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