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CCNA Practice Question: An engineer is troubleshooting an OSPFv3…

This 200-301 practice question tests your understanding of an engineer is troubleshooting an ospfv3…. 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.

Exhibit

R1# show running-config | section router ospfv3
router ospfv3 1
 router-id 1.1.1.1
 area 0 authentication ipsec spi 256 md5 1234567890ABCDEF1234567890ABCDEF
!
interface Serial0/0/0
 ipv6 address 2001:DB8:1:1::1/64
 ospfv3 1 ipv6 area 0

R2# show running-config | section router ospfv3
router ospfv3 1
 router-id 2.2.2.2
!
interface Serial0/0/0
 ipv6 address 2001:DB8:1:1::2/64
 ospfv3 1 ipv6 area 0

An engineer is troubleshooting an OSPFv3 adjacency issue between two routers R1 and R2 connected over a serial link. The link is up/up on both sides, and IPv6 is enabled on the interfaces. However, the 'show ipv6 ospf neighbor' command shows no neighbors. The engineer checks the OSPFv3 configuration. What is the most likely cause of the missing adjacency?

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.

Question 1hardmultiple choice
Full question →

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

✓

OSPFv3 authentication is configured on R1 but not on R2.

The root cause is that R1 has OSPFv3 authentication configured under the router ospfv3 process using IPsec, but R2 does not have any authentication configured. OSPFv3 authentication must match on both sides for an adjacency to form. The 'show ip ospf neighbor' command shows no neighbors because the authentication mismatch prevents Hello packets from being accepted. The correct fix is to configure matching authentication on R2 or remove it from R1. Option B is correct because it addresses the mismatch. Option A is not the cause because the interfaces are up. Option C is not the cause because OSPFv3 uses link-local addresses, not the global unicast address. Option D is not the cause because the OSPFv3 process is configured with an explicit router-id.

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 serial interface on R2 is administratively down.

    Why it's wrong here

    The exhibit shows the interface is up/up on both sides, so this is not the issue.

  • ✓

    OSPFv3 authentication is configured on R1 but not on R2.

    Why this is correct

    R1 has IPsec authentication under router ospfv3, but R2 does not. This mismatch prevents adjacency formation.

    Clue confirmation

    The clue word "most likely" in the question point toward this answer.

    Related concept

    OSPF neighbours must agree on key parameters.

  • ✗

    The IPv6 address on R2 is in a different subnet than R1.

    Why it's wrong here

    Both interfaces have addresses in the 2001:DB8:1:1::/64 subnet, so the subnet matches.

  • ✗

    The OSPFv3 router-id is not configured on R2.

    Why it's wrong here

    R2 has a router-id of 2.2.2.2 configured, so this is not the problem.

Option-by-option analysis

Why each answer is right or wrong

Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.

✓OSPFv3 authentication is configured on R1 but not on R2.Correct answer▾

Why this is correct

R1 has IPsec authentication under router ospfv3, but R2 does not. This mismatch prevents adjacency formation.

✗The serial interface on R2 is administratively down.Wrong answer — click to see why▾

Why this is wrong here

The symptom indicates a missing adjacency despite the interface being operational.

✗The IPv6 address on R2 is in a different subnet than R1.Wrong answer — click to see why▾

Why this is wrong here

OSPFv3 adjacency uses link-local addresses, not global unicast, so subnet mismatch is not the issue.

✗The OSPFv3 router-id is not configured on R2.Wrong answer — click to see why▾

Why this is wrong here

A router-id is present; missing router-id would not prevent adjacency but would cause other issues.

Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”

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

    The exhibit shows the interface is up/up on both sides, so this is not the issue.

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.

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 200-301 OSPF questions on adjacency and route selection.

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FAQ

Questions learners often ask

What does this 200-301 question test?

OSPF neighbours must agree on key parameters.

What is the correct answer to this question?

The correct answer is: OSPFv3 authentication is configured on R1 but not on R2. — The root cause is that R1 has OSPFv3 authentication configured under the router ospfv3 process using IPsec, but R2 does not have any authentication configured. OSPFv3 authentication must match on both sides for an adjacency to form. The 'show ip ospf neighbor' command shows no neighbors because the authentication mismatch prevents Hello packets from being accepted. The correct fix is to configure matching authentication on R2 or remove it from R1. Option B is correct because it addresses the mismatch. Option A is not the cause because the interfaces are up. Option C is not the cause because OSPFv3 uses link-local addresses, not the global unicast address. Option D is not the cause because the OSPFv3 process is configured with an explicit router-id.

What should I do if I get this 200-301 question wrong?

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 200-301 OSPF questions on adjacency and route selection.

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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