hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting IPv6 routing issues between…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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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Written by Johnson Ajibi, MSc IT Security
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