- A
The summary-address command in OSPFv3 requires a prefix-length, not a mask; the engineer used a /64 prefix, which is too specific to summarize the /80 subnet.
Correct. The summary-address command in OSPFv3 uses a prefix length. A /64 summary will not include a /80 subnet because the /80 is more specific and falls outside the summary range.
- B
The OSPFv3 process is not enabled on the interface facing the 2001:db8:0:1:1::/80 subnet.
Why wrong: Incorrect. The issue is with summarization, not with OSPFv3 enablement.
- C
The summary-address command should be applied on the interface, not under the OSPFv3 process.
Why wrong: Incorrect. In OSPFv3, the summary-address command is configured under the router ospfv3 process.
- D
The 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol.
Why wrong: Incorrect. The summary-address command works for redistributed routes as well.
Quick Answer
The answer is that the engineer used a /64 prefix-length in the summary-address command, which is too specific to summarize the /80 subnet. In OSPFv3, the summary-address command requires a prefix-length, not a subnet mask, and the specified prefix must be less specific (shorter) than the component routes it is meant to summarize. A /64 summary cannot cover a /80 component route because the summary prefix must encompass the longer prefixes; instead, a shorter prefix like /56 or /48 would be needed. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of OSPFv3 summarization syntax and the critical distinction between IPv4 masks and IPv6 prefix-lengths—a common trap is confusing the two or assuming a /64 automatically summarizes all subnets under it. Remember the memory tip: “Summarize shorter, not longer; a /64 won’t cover an /80.”
300-410 Route Summarization Practice Question
This 300-410 practice question tests your understanding of route summarization. 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 a route summarization issue in a network using OSPFv3. Router R1 is configured with the 'ipv6 summary-address 2001:db8:0:1::/64' command under the OSPFv3 process. After the configuration, routers in the same area lose connectivity to the 2001:db8:0:1:1::/80 subnet, which is one of the component 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.
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 summary-address command in OSPFv3 requires a prefix-length, not a mask; the engineer used a /64 prefix, which is too specific to summarize the /80 subnet.
In OSPFv3, the summary-address command works similarly to OSPFv2. The issue may be that the summary route is not being generated because the component routes are not in the OSPFv3 database, or the summary mask is too specific.
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 summary-address command in OSPFv3 requires a prefix-length, not a mask; the engineer used a /64 prefix, which is too specific to summarize the /80 subnet.
Why this is correct
Correct. The summary-address command in OSPFv3 uses a prefix length. A /64 summary will not include a /80 subnet because the /80 is more specific and falls outside the summary range.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The OSPFv3 process is not enabled on the interface facing the 2001:db8:0:1:1::/80 subnet.
Why it's wrong here
Incorrect. The issue is with summarization, not with OSPFv3 enablement.
- ✗
The summary-address command should be applied on the interface, not under the OSPFv3 process.
Why it's wrong here
Incorrect. In OSPFv3, the summary-address command is configured under the router ospfv3 process.
- ✗
The 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol.
Why it's wrong here
Incorrect. The summary-address command works for redistributed routes as well.
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. In OSPFv3, the summary-address command is configured under the router ospfv3 process.
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 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?
Route Summarization — This question tests Route Summarization — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The summary-address command in OSPFv3 requires a prefix-length, not a mask; the engineer used a /64 prefix, which is too specific to summarize the /80 subnet. — In OSPFv3, the summary-address command works similarly to OSPFv2. The issue may be that the summary route is not being generated because the component routes are not in the OSPFv3 database, or the summary mask is too specific.
What should I do if I get this 300-410 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 300-410 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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Last reviewed: Jun 18, 2026
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