Question 272 of 2,152

Route Summarization →hardMultiple ChoiceObjective-mapped

OSPFv3 Route Summarization Prefix-Length Issue

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.

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.
Why wrong: Incorrect. A /64 summary includes all subnets within that range, including a /80 subnet. This would not cause loss of connectivity.
B
The OSPFv3 process is not enabled on the interface facing the 2001:db8:0:1:1::/80 subnet.
Why wrong: Incorrect. If the OSPFv3 process were not enabled on the interface, the route would not be learned at all, not just after summarization.
C
The summary-address command should be applied on the interface, not under the OSPFv3 process.
Why wrong: Incorrect. The summary-address command is correctly configured under the OSPFv3 process, not on the interface.
D
The 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol.
Correct. The /80 subnet is redistributed (external) and the summary-address command aggregates it, suppressing the specific route and causing loss of connectivity.

Full breakdown with real-world context →

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.”

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 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol.

The 'ipv6 summary-address' command in OSPFv3 is used to summarize external routes (type 5 and 7 LSAs). If the /80 subnet is redistributed from another protocol, it is an external route. However, the summary-address command will create an aggregate route that suppresses more specific external routes. After configuring the /64 summary, the /80 route is no longer advertised individually, causing connectivity loss because only the aggregate exists. The issue is that the component route is external and the summary command suppresses it, not that the prefix length 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 it's wrong here
Incorrect. A /64 summary includes all subnets within that range, including a /80 subnet. This would not cause loss of connectivity.
✗
The OSPFv3 process is not enabled on the interface facing the 2001:db8:0:1:1::/80 subnet.
Why it's wrong here
Incorrect. If the OSPFv3 process were not enabled on the interface, the route would not be learned at all, not just after summarization.
✗
The summary-address command should be applied on the interface, not under the OSPFv3 process.
Why it's wrong here
Incorrect. The summary-address command is correctly configured under the OSPFv3 process, not on the interface.
✓
The 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol.
Why this is correct
Correct. The /80 subnet is redistributed (external) and the summary-address command aggregates it, suppressing the specific route and causing loss of connectivity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.

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 summary-address command is correctly configured under the OSPFv3 process, not on the interface.

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?

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 2001:db8:0:1:1::/80 subnet is not in the OSPFv3 database because it is redistributed from another protocol. — The 'ipv6 summary-address' command in OSPFv3 is used to summarize external routes (type 5 and 7 LSAs). If the /80 subnet is redistributed from another protocol, it is an external route. However, the summary-address command will create an aggregate route that suppresses more specific external routes. After configuring the /64 summary, the /80 route is no longer advertised individually, causing connectivity loss because only the aggregate exists. The issue is that the component route is external and the summary command suppresses it, not that the prefix length is too specific.

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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