Question 1,546 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Strict Mode Drops OSPFv3 Hellos — Link-Local Source

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.

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.

Full breakdown with real-world context →

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.”

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.

IPv6 uRPF strict mode checks that the source IPv6 address of an incoming packet matches a route in the routing table and that the incoming interface is the same as the outgoing interface for that route. OSPFv3 Hello packets are sent with the link-local address of the sending interface as the source IPv6 address. If that link-local address is not present in the IPv6 routing table (e.g., because OSPFv3 has not yet installed the neighbor's link-local route), uRPF strict mode drops the packet, preventing the adjacency from forming.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

Cisco often tests the misconception that uRPF drops multicast traffic by default, but the real issue is that OSPFv3's link-local source addresses are not yet in the routing table when strict mode is applied, causing the adjacency to fail.

Detailed technical explanation

How to think about this question

Under the hood, IPv6 uRPF strict mode leverages the Forwarding Information Base (FIB) to perform the reachability and interface check. In OSPFv3, the neighbor's link-local address is learned via the Hello process, but until the adjacency is established, that address may not be in the FIB, creating a chicken-and-egg problem. A real-world workaround is to configure 'ipv6 verify unicast source reachable-via any' (loose mode) on the interface, or to ensure that the link-local address is reachable via a static route or the connected route of the interface itself.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

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.

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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 — Read the scenario before looking for a memorised answer..

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. — IPv6 uRPF strict mode checks that the source IPv6 address of an incoming packet matches a route in the routing table and that the incoming interface is the same as the outgoing interface for that route. OSPFv3 Hello packets are sent with the link-local address of the sending interface as the source IPv6 address. If that link-local address is not present in the IPv6 routing table (e.g., because OSPFv3 has not yet installed the neighbor's link-local route), uRPF strict mode drops the packet, preventing the adjacency from forming.

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

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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