Question 989 of 2,152

OSPF Troubleshooting (v2/v3) →hardMultiple ChoiceObjective-mapped

OSPFv3 No IPv6 Address on Interface

This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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 OSPFv3 on a dual-stack network. Routers R1 and R2 are connected via a serial link. Both routers have OSPFv3 configured for IPv6. The engineer runs 'show ipv6 ospf neighbor' on R1 and sees R2 as FULL/DR. However, R1 cannot ping the IPv6 address of R2's loopback interface. 'show ipv6 route ospf' on R1 does not show any OSPF routes. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.
Correct. 'no ipv6 unicast-routing' prevents R1 from installing any IPv6 routes, including OSPF-learned routes, even though OSPF adjacency is established.
B
The interface on R2 is configured as passive under OSPFv3.
Why wrong: Incorrect. If the interface on R2 were passive, it would either break adjacency (if on the serial link) or still advertise the loopback prefix (if on the loopback). Adjacency is FULL, so this is not the cause.
C
The interface on R1 does not have an IPv6 address configured.
Why wrong: Incorrect. OSPFv3 uses link-local addresses for neighbor communication; a global IPv6 address on the interface is not required to learn routes. The adjacency is FULL, so the interface has at least a link-local address.
D
The OSPFv3 process is configured with 'default-information originate always' but no default route exists.
Why wrong: Incorrect. The 'default-information originate always' command only affects the advertisement of a default route; it does not explain the absence of all OSPF routes.

Full breakdown with real-world context →

Quick Answer

The answer is that the interface on R1 does not have an IPv6 address configured. In OSPFv3, an IPv6 address must be present on the interface for the router to advertise connected prefixes into the OSPFv3 database; without it, the router can form a neighbor adjacency (as seen with the FULL/DR state) but cannot install any OSPFv3 routes because there is no local IPv6 prefix to originate or receive. This scenario tests your understanding that OSPFv3 adjacency and route installation are separate processes—a common trap on the Cisco CCNP ENARSI 300-410 exam, where engineers mistakenly focus on network type or DR/BDR issues when the real problem is a missing IPv6 address on the interface. Remember the memory tip: “No IPv6 address, no OSPFv3 route—adjacency is just a handshake, not a route table update.”

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 OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.

The correct answer is A. Even though the OSPFv3 neighbor adjacency is FULL/DR, the global command 'no ipv6 unicast-routing' on R1 disables IPv6 routing. As a result, R1 does not install any IPv6 routes into the routing table, including OSPF-learned routes. This explains why 'show ipv6 route ospf' shows no routes and R1 cannot ping R2's loopback. Option C is incorrect because OSPFv3 only requires a link-local address on the interface to establish adjacency; a global IPv6 address is not necessary for route advertisement. Options B and D do not fit the symptoms.

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.

✓
The OSPFv3 process has 'no ipv6 unicast-routing' enabled globally.
Why this is correct
Correct. 'no ipv6 unicast-routing' prevents R1 from installing any IPv6 routes, including OSPF-learned routes, even though OSPF adjacency is established.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The interface on R2 is configured as passive under OSPFv3.
Why it's wrong here
Incorrect. If the interface on R2 were passive, it would either break adjacency (if on the serial link) or still advertise the loopback prefix (if on the loopback). Adjacency is FULL, so this is not the cause.
✗
The interface on R1 does not have an IPv6 address configured.
Why it's wrong here
Incorrect. OSPFv3 uses link-local addresses for neighbor communication; a global IPv6 address on the interface is not required to learn routes. The adjacency is FULL, so the interface has at least a link-local address.
✗
The OSPFv3 process is configured with 'default-information originate always' but no default route exists.
Why it's wrong here
Incorrect. The 'default-information originate always' command only affects the advertisement of a default route; it does not explain the absence of all OSPF routes.

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.

Trap categories for this question

Command / output trap
Incorrect. The 'default-information originate always' command only affects the advertisement of a default route; it does not explain the absence of all OSPF routes.

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.

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

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?

OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: The OSPFv3 process has 'no ipv6 unicast-routing' enabled globally. — The correct answer is A. Even though the OSPFv3 neighbor adjacency is FULL/DR, the global command 'no ipv6 unicast-routing' on R1 disables IPv6 routing. As a result, R1 does not install any IPv6 routes into the routing table, including OSPF-learned routes. This explains why 'show ipv6 route ospf' shows no routes and R1 cannot ping R2's loopback. Option C is incorrect because OSPFv3 only requires a link-local address on the interface to establish adjacency; a global IPv6 address is not necessary for route advertisement. Options B and D do not fit the symptoms.

What should I do if I get this 300-410 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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