- A
The OSPFv3 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them.
Correct. OSPFv3 uses link-local source addresses, which are not globally routable and not present in the routing table, so uRPF strict mode drops them.
- B
The OSPFv3 Hello packets are multicast to ff02::5, and uRPF strict mode drops all multicast traffic by default.
Why wrong: Incorrect. uRPF strict mode does not specifically drop multicast traffic; it checks source address reachability regardless of destination.
- C
The OSPFv3 Hello packets have a hop limit of 1, and uRPF strict mode requires a hop limit of at least 2.
Why wrong: Incorrect. uRPF does not inspect hop limit; it only verifies source address reachability.
- D
The interface has IPv6 unicast-routing disabled, which prevents uRPF from functioning correctly.
Why wrong: Incorrect. IPv6 unicast-routing must be enabled globally, but this alone does not cause uRPF to drop OSPFv3 Hellos.
Quick Answer
The answer is that uRPF strict mode drops OSPFv3 Hellos because these packets use link-local source addresses (fe80::), which are not present in the global routing table. When uRPF strict mode is enabled on an interface, it verifies that the source address of every incoming packet is reachable via that exact interface according to the routing table. Since OSPFv3 Hellos are sourced from a link-local address, and link-local addresses are not installed in the IPv6 routing table, the strict mode check fails and the packets are dropped, breaking the OSPFv3 neighbor adjacency. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the interaction between uRPF and routing protocols—a common trap is forgetting that uRPF strict mode applies to all incoming traffic, including control plane packets. A helpful memory tip: “Strict uRPF needs a route; link-local Hellos are left out.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
An engineer configures IPv6 uRPF strict mode on an interface of a router that participates in OSPFv3. The router starts dropping OSPFv3 Hello packets received on that interface, causing the OSPFv3 neighbor adjacency to fail. Which is the most likely explanation?
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 OSPFv3 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them.
OSPFv3 uses link-local addresses (fe80::) for neighbor communication. uRPF strict mode checks that the source address of incoming packets is reachable via the incoming interface in the routing table. Link-local addresses are not typically installed in the global routing table, so uRPF strict mode drops them, breaking OSPFv3 adjacency.
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 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them.
Why this is correct
Correct. OSPFv3 uses link-local source addresses, which are not globally routable and not present in the routing table, so uRPF strict mode drops them.
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 Hello packets are multicast to ff02::5, and uRPF strict mode drops all multicast traffic by default.
Why it's wrong here
Incorrect. uRPF strict mode does not specifically drop multicast traffic; it checks source address reachability regardless of destination.
- ✗
The OSPFv3 Hello packets have a hop limit of 1, and uRPF strict mode requires a hop limit of at least 2.
Why it's wrong here
Incorrect. uRPF does not inspect hop limit; it only verifies source address reachability.
- ✗
The interface has IPv6 unicast-routing disabled, which prevents uRPF from functioning correctly.
Why it's wrong here
Incorrect. IPv6 unicast-routing must be enabled globally, but this alone does not cause uRPF to drop OSPFv3 Hellos.
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.
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?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The OSPFv3 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them. — OSPFv3 uses link-local addresses (fe80::) for neighbor communication. uRPF strict mode checks that the source address of incoming packets is reachable via the incoming interface in the routing table. Link-local addresses are not typically installed in the global routing table, so uRPF strict mode drops them, breaking OSPFv3 adjacency.
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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