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300-410 Practice Question: A large enterprise network uses OSPFv3 for IPv6…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
Why it's wrong here
MTU mismatch typically causes 'Bad LSA' or adjacency stuck in EXSTART, not specifically 'Bad LSReq'.
- ✗
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.
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
IPv6 Next Hop Resolution
IPv6 Next Hop Resolution is the process a router uses to determine the next router or destination device to which a packet should be forwarded, based on the destination IPv6 address and the routing table.
Key term
IPv6 Routing OSPFv3
OSPFv3 is a routing protocol that lets routers share information about how to reach destinations in an IPv6 network, updating routes automatically as the network changes.
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