- 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.
OSPFv3 Network Type Mismatch and ACL: Point-to-Point vs Broadcast with ACL Blocking Multicast
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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?
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.”
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.
R1's inbound IPv6 ACL permits only OSPF (protocol 89) but denies all other traffic. When R1's interface is configured as 'ipv6 ospf network point-to-point', it expects to send unicast OSPFv3 packets to the neighbor, not multicast. However, R2, using the default broadcast network type, sends OSPFv3 hellos to the multicast address FF02::5 (AllSPFRouters). R1's ACL blocks these multicast hellos because they are not protocol 89 unicast packets, preventing adjacency formation. The 'Bad LSReq' error in EXSTART/EXCHANGE indicates that R1 never received R2's hellos, so it cannot build a proper link-state database.
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 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
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
Cisco often tests the interaction between OSPF network types and ACLs, where candidates assume that permitting OSPF protocol 89 is sufficient, forgetting that multicast packets (FF02::5) may be dropped if the ACL does not explicitly allow them or if the interface is configured for unicast-only communication.
Detailed technical explanation
How to think about this question
OSPFv3 uses multicast addresses FF02::5 (AllSPFRouters) and FF02::6 (AllDRouters) for broadcast network types. When an interface is set to point-to-point, OSPFv3 sends unicast packets to the neighbor's link-local address, bypassing DR/BDR election. An ACL that permits only protocol 89 (OSPF) but does not explicitly permit multicast traffic (e.g., permit 89 any any) will still drop multicast packets because the ACL processes the packet's destination address; multicast packets are not matched by a simple protocol permit unless the ACL also allows the multicast group. In real-world scenarios, this misconfiguration often occurs when engineers apply ACLs without considering the OSPF network type.
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?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
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. — R1's inbound IPv6 ACL permits only OSPF (protocol 89) but denies all other traffic. When R1's interface is configured as 'ipv6 ospf network point-to-point', it expects to send unicast OSPFv3 packets to the neighbor, not multicast. However, R2, using the default broadcast network type, sends OSPFv3 hellos to the multicast address FF02::5 (AllSPFRouters). R1's ACL blocks these multicast hellos because they are not protocol 89 unicast packets, preventing adjacency formation. The 'Bad LSReq' error in EXSTART/EXCHANGE indicates that R1 never received R2's hellos, so it cannot build a proper link-state database.
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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