- A
The OSPF network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops.
R1's point-to-point configuration means it does not participate in DR election, so R2 becomes DR. R1 treats the link as a point-to-point link and may not advertise the correct type 2 LSA, while R2 advertises a type 2 LSA. This mismatch can cause R1 to learn routes via R2 and R2 to learn routes via R1, creating a loop.
- B
The IP addresses are on the same subnet, but the OSPF cost is set differently, causing unequal cost load balancing.
Why wrong: Cost mismatch would not cause a loop; it would affect path selection.
- C
The OSPF hello and dead intervals are mismatched due to the network type, causing the adjacency to flap.
Why wrong: The adjacency is FULL, so hello/dead intervals are not mismatched.
- D
The OSPF area is misconfigured; one router is in a different area.
Why wrong: Both are in area 0, as stated.
Quick Answer
The root cause is an OSPF network type mismatch between R1’s point-to-point and R2’s broadcast configuration, which creates a routing loop. When one router expects a point-to-point link (no DR/BDR election) and the other expects a broadcast link (with DR/BDR election), they still form a FULL adjacency but with conflicting topological views. R1 treats the link as a stub network and may advertise a route back through R2, while R2, seeing itself as the DR, advertises the link as a transit network; this inconsistency causes each router to believe the other is the next hop for the same subnet, looping traffic between them. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how OSPF network types affect LSA generation and neighbor states—a common trap is assuming mismatched types prevent adjacency entirely, but they often form FULL with a DR/DROTHER mismatch. Remember: on multi-access links, point-to-point and broadcast are not interchangeable; always match the network type to avoid silent loops.
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. 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 is experiencing routing loops between two routers. R1 has the following configuration: interface GigabitEthernet0/0 ip address 10.1.1.1 255.255.255.0 ip ospf network point-to-point. R2 has: interface GigabitEthernet0/0 ip address 10.1.1.2 255.255.255.0 ip ospf network broadcast. Both routers are in area 0. R1 shows: 'show ip ospf neighbor' lists R2 as FULL/DR, but R2 shows R1 as FULL/DROTHER. Traffic between two subnets behind each router is looping. What is the root cause?
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
The OSPF network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops.
OSPF network type mismatch on a multi-access link can cause adjacency issues. R1 is configured as point-to-point, which does not participate in DR/BDR election and expects a point-to-point link. R2 is configured as broadcast, which expects a DR/BDR election. This mismatch can cause both routers to form an adjacency (since OSPF still works), but they have different views of the network. R1 thinks the link is point-to-point and does not expect a DR, so it may advertise the link as a stub network or with different metrics. R2 thinks it is the DR (since R1 is not participating in election), and it advertises the link as a transit network. This inconsistency can cause routing loops because R1 may advertise a route via R2, and R2 may advertise the same route via R1, creating a loop. The correct fix is to match the network type on both ends.
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 OSPF network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops.
Why this is correct
R1's point-to-point configuration means it does not participate in DR election, so R2 becomes DR. R1 treats the link as a point-to-point link and may not advertise the correct type 2 LSA, while R2 advertises a type 2 LSA. This mismatch can cause R1 to learn routes via R2 and R2 to learn routes via R1, creating a loop.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The IP addresses are on the same subnet, but the OSPF cost is set differently, causing unequal cost load balancing.
Why it's wrong here
Cost mismatch would not cause a loop; it would affect path selection.
- ✗
The OSPF hello and dead intervals are mismatched due to the network type, causing the adjacency to flap.
Why it's wrong here
The adjacency is FULL, so hello/dead intervals are not mismatched.
- ✗
The OSPF area is misconfigured; one router is in a different area.
Why it's wrong here
Both are in area 0, as stated.
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.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The OSPF network type mismatch causes both routers to become DR/BDR, leading to incorrect LSA generation and routing loops. — OSPF network type mismatch on a multi-access link can cause adjacency issues. R1 is configured as point-to-point, which does not participate in DR/BDR election and expects a point-to-point link. R2 is configured as broadcast, which expects a DR/BDR election. This mismatch can cause both routers to form an adjacency (since OSPF still works), but they have different views of the network. R1 thinks the link is point-to-point and does not expect a DR, so it may advertise the link as a stub network or with different metrics. R2 thinks it is the DR (since R1 is not participating in election), and it advertises the link as a transit network. This inconsistency can cause routing loops because R1 may advertise a route via R2, and R2 may advertise the same route via R1, creating a loop. The correct fix is to match the network type on both ends.
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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Last reviewed: Jun 18, 2026
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