- A
The OSPFv3 process ID does not match between R1 and R2.
Why wrong: OSPFv3 process IDs are locally significant; they do not need to match.
- B
The serial interface has a mismatched network type, such as point-to-multipoint, which prevents adjacency formation on a point-to-point link.
A network type mismatch can cause Hello packets to be ignored or not processed correctly, leading to INIT state.
- C
The IPv6 address on the serial interface is not in the same subnet as R2's address.
Why wrong: Serial links typically use link-local addresses; global addresses are not required for adjacency.
- D
The OSPFv3 hello interval is set to a non-default value that is not supported on serial links.
Why wrong: Hello intervals can be adjusted; the issue is more fundamental.
Question 698 of 2,152
Resolve OSPFv3 Adjacency Stuck in INIT on Serial Links Due to Network Type Mismatch
This 300-410 practice question tests your understanding of ipv6 first hop security. 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 network engineer is troubleshooting IPv6 routing issues between two routers connected via a serial link. Router R1 and Router R2 are running OSPFv3. The OSPFv3 adjacency is not forming. Router R1 has the following relevant configuration:
interface Serial0/0
ipv6 address 2001:DB8:1::1/64 ipv6 ospf 1 area 0 !
Router R2 shows: debug ipv6 ospf hello output indicates that R2 is receiving Hello packets from R1, but the neighbor state remains INIT. What is the root cause?
Quick Answer
The answer is a mismatched OSPFv3 network type on the serial link, such as point-to-multipoint, which prevents the adjacency from progressing past INIT. On point-to-point serial interfaces, OSPFv3 defaults to the point-to-point network type, which uses link-local addresses for neighbor discovery and expects a direct two-way handshake. If one router is configured with a non-default network type like point-to-multipoint, it will not send the correct Database Description packets or may require a designated router election, causing the receiving router to see Hello packets but remain stuck in INIT. This scenario tests your understanding of OSPFv3 adjacency mechanics and network type behavior, a common trap on the Cisco CCNP ENARSI 300-410 exam where engineers overlook that serial links are not always point-to-point by default. A quick memory tip: if you see INIT but hear Hellos, check the network type—point-to-point needs no DR, so any other type breaks the handshake.
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 serial interface has a mismatched network type, such as point-to-multipoint, which prevents adjacency formation on a point-to-point link.
On a point-to-point serial link, OSPFv3 defaults to the point-to-point network type. If one side is configured as point-to-multipoint (or another non-broadcast type), the routers will not form a full adjacency because the Hello protocol and neighbor discovery mechanisms differ. The debug output shows R2 receives Hellos from R1 but stays in INIT, which indicates that R2 does not see its own Router ID in the Hello packet's neighbor list — a classic symptom of network type mismatch on a serial interface.
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.
- ✗
The OSPFv3 process ID does not match between R1 and R2.
Why it's wrong here
OSPFv3 process IDs are locally significant; they do not need to match.
- ✓
The serial interface has a mismatched network type, such as point-to-multipoint, which prevents adjacency formation on a point-to-point link.
Why this is correct
A network type mismatch can cause Hello packets to be ignored or not processed correctly, leading to INIT state.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The IPv6 address on the serial interface is not in the same subnet as R2's address.
Why it's wrong here
Serial links typically use link-local addresses; global addresses are not required for adjacency.
- ✗
The OSPFv3 hello interval is set to a non-default value that is not supported on serial links.
Why it's wrong here
Hello intervals can be adjusted; the issue is more fundamental.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that OSPFv3 requires matching global IPv6 subnets or process IDs, when in fact the adjacency is built using link-local addresses and the process ID is locally significant.
Detailed technical explanation
How to think about this question
OSPFv3 relies on link-local addresses (FE80::/10) for neighbor discovery and uses the 'ipv6 ospf network' command to set the network type. On serial links, the default is point-to-point, which suppresses DR/BDR election and expects a single neighbor. If one side is set to point-to-multipoint, the router expects to see its own Router ID in received Hellos before advancing past INIT, but point-to-multipoint Hellos do not include the neighbor's Router ID in the same way, causing the state to stall at INIT. This behavior is defined in RFC 5340 and is a common misconfiguration when migrating from OSPFv2.
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 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The serial interface has a mismatched network type, such as point-to-multipoint, which prevents adjacency formation on a point-to-point link. — On a point-to-point serial link, OSPFv3 defaults to the point-to-point network type. If one side is configured as point-to-multipoint (or another non-broadcast type), the routers will not form a full adjacency because the Hello protocol and neighbor discovery mechanisms differ. The debug output shows R2 receives Hellos from R1 but stays in INIT, which indicates that R2 does not see its own Router ID in the Hello packet's neighbor list — a classic symptom of network type mismatch on a serial interface.
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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