- A
R1's ACL blocks OSPFv3 multicast hello packets (FF02::5), preventing adjacency formation on the broadcast link.
On a broadcast network, OSPFv3 sends hellos to FF02::5. R1's ACL permits only OSPF protocol, but the destination address is filtered because the ACL does not explicitly permit multicast. The network type mismatch exacerbates the issue as R1 expects unicast hellos.
- B
R2's OSPFv3 process is configured with a different router-id, causing a DR election conflict.
Why wrong: Router-id mismatch would not cause 'Bad LSReq' specifically; it would affect neighbor state earlier.
- C
The MTU mismatch between R1 and R2 causes OSPFv3 packet fragmentation, leading to LSReq errors.
Why wrong: MTU mismatch typically causes 'Bad LSA' or adjacency stuck in EXSTART, not specifically 'Bad LSReq'.
- D
R2 has a passive interface configured, preventing it from sending OSPFv3 hellos.
Why wrong: A passive interface would prevent neighbor discovery entirely, not cause EXSTART/EXCHANGE state.
Quick Answer
The root cause is the ACL on R1 blocking OSPFv3 multicast hello packets (FF02::5), which prevents adjacency formation on the broadcast link. This occurs because R1’s point-to-point network type uses unicast hellos, while R2’s default broadcast type relies on multicast FF02::5 for neighbor discovery; the inbound ACL on R1 permits only OSPF protocol 89 but does not explicitly allow the multicast destination, so R1 never receives R2’s hellos, stalling the adjacency in EXSTART/EXCHANGE with the “Bad LSReq” log. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPFv3 network type behavior and how ACLs interact with multicast versus unicast control plane traffic—a common trap is assuming protocol 89 alone covers all OSPF packets, forgetting that multicast addressing is a separate filter criterion. Memory tip: “Point-to-point talks unicast, broadcast shouts multicast—if your ACL only permits the protocol, it still silences the shout.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
A large enterprise network uses OSPFv3 for IPv6 routing. Router R1 and R2 are connected via a multi-access Ethernet link. R1 is configured with 'ipv6 ospf network point-to-point' while R2 uses the default broadcast network type. R1 has an IPv6 ACL applied inbound on its interface that permits only OSPF (89) and denies all other traffic. R2 is unable to form a full OSPF adjacency with R1. R2 shows 'OSPFv3 adjacency state is EXSTART/EXCHANGE' and logs 'Bad LSReq'. 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
R1's ACL blocks OSPFv3 multicast hello packets (FF02::5), preventing adjacency formation on the broadcast link.
The network type mismatch between R1 (point-to-point) and R2 (broadcast) causes different DR/BDR election behavior and LSDB synchronization issues. The ACL on R1 blocks the multicast traffic used for OSPFv3 hello packets (FF02::5) on broadcast networks, but point-to-point uses unicast. The 'Bad LSReq' indicates that the LS Request packets are being filtered or malformed due to the mismatch.
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.
- ✓
R1's ACL blocks OSPFv3 multicast hello packets (FF02::5), preventing adjacency formation on the broadcast link.
Why this is correct
On a broadcast network, OSPFv3 sends hellos to FF02::5. R1's ACL permits only OSPF protocol, but the destination address is filtered because the ACL does not explicitly permit multicast. The network type mismatch exacerbates the issue as R1 expects unicast hellos.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
R2's OSPFv3 process is configured with a different router-id, causing a DR election conflict.
Why it's wrong here
Router-id mismatch would not cause 'Bad LSReq' specifically; it would affect neighbor state earlier.
- ✗
The MTU mismatch between R1 and R2 causes OSPFv3 packet fragmentation, leading to LSReq errors.
- ✗
R2 has a passive interface configured, preventing it from sending OSPFv3 hellos.
Why it's wrong here
A passive interface would prevent neighbor discovery entirely, not cause EXSTART/EXCHANGE state.
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?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: R1's ACL blocks OSPFv3 multicast hello packets (FF02::5), preventing adjacency formation on the broadcast link. — The network type mismatch between R1 (point-to-point) and R2 (broadcast) causes different DR/BDR election behavior and LSDB synchronization issues. The ACL on R1 blocks the multicast traffic used for OSPFv3 hello packets (FF02::5) on broadcast networks, but point-to-point uses unicast. The 'Bad LSReq' indicates that the LS Request packets are being filtered or malformed due to the mismatch.
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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