Question 91 of 2,152

VRF-Lite →hardMultiple ChoiceObjective-mapped

uRPF Strict Mode in VRF-Lite

This 300-410 practice question tests your understanding of vrf-lite. 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.

A router is configured with uRPF (Unicast Reverse Path Forwarding) in strict mode on an interface that belongs to a VRF. The network uses asymmetric routing for load balancing. The engineer notices that legitimate traffic from a customer is being dropped. 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 uRPF strict mode requires that the source IP address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface.
Strict uRPF fails if the best route to the source is not via the incoming interface. Asymmetric routing violates this assumption.
B
The uRPF loose mode is configured instead of strict mode, which only checks that a route exists for the source IP, not the interface.
Why wrong: Loose mode would allow the traffic, but the question states strict mode is configured.
C
The VRF has a default route that points to the incoming interface, causing uRPF to always succeed.
Why wrong: A default route would make uRPF succeed, not fail.
D
The 'ip verify unicast source reachable-via any' command is used, which is the loose mode, not strict.
Why wrong: This is loose mode, not strict.

Full breakdown with real-world context →

Quick Answer

The answer is that uRPF strict mode drops the traffic because it requires the source IP address to be reachable via the same interface on which the packet arrived, but asymmetric routing in a VRF-Lite environment sends the return path out a different interface. Strict mode performs a reverse path lookup against the VRF’s routing table, and if the best route for the source IP does not point back to the ingress interface, the packet is discarded—even if the traffic is legitimate. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how uRPF interacts with VRF-Lite and non-symmetric topologies; a common trap is assuming strict mode works like loose mode, which only checks for any valid route. Remember the memory tip: “Strict is strict—same interface in, same interface out; loose is loose—any route will do.”

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 uRPF strict mode requires that the source IP address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface.

Strict uRPF checks that the source IP address of incoming packets has a route in the routing table that points back to the same interface. In asymmetric routing, the return path may be different (e.g., out another interface), so the source IP may not have a route back to the incoming interface. This causes legitimate traffic to be dropped. The edge case is that uRPF strict mode does not account for asymmetric routing, while loose mode does.

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 uRPF strict mode requires that the source IP address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface.
Why this is correct
Strict uRPF fails if the best route to the source is not via the incoming interface. Asymmetric routing violates this assumption.
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 uRPF loose mode is configured instead of strict mode, which only checks that a route exists for the source IP, not the interface.
Why it's wrong here
Loose mode would allow the traffic, but the question states strict mode is configured.
✗
The VRF has a default route that points to the incoming interface, causing uRPF to always succeed.
Why it's wrong here
A default route would make uRPF succeed, not fail.
✗
The 'ip verify unicast source reachable-via any' command is used, which is the loose mode, not strict.
Why it's wrong here
This is loose mode, not strict.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

Quick reference

Asymmetric Encryption Algorithm Comparison

Algorithm	Key Exchange	Signatures	Equivalent Security Key	Notes
RSA-3072	Yes	Yes	128-bit	Widely deployed; slow for bulk data
ECDSA P-256	No	Yes	128-bit	Fast signatures; standard TLS certs
ECDH / ECDHE	Yes	No	128-bit	Perfect forward secrecy in TLS 1.3
DH / DHE	Yes	No	128-bit (3072-bit key)	Replaced by ECDHE in modern TLS
Ed25519	No	Yes	~128-bit	SSH keys, modern PKI

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

VRF-Lite — This question tests VRF-Lite — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The uRPF strict mode requires that the source IP address be reachable via the same interface, but asymmetric routing causes the return path to use a different interface. — Strict uRPF checks that the source IP address of incoming packets has a route in the routing table that points back to the same interface. In asymmetric routing, the return path may be different (e.g., out another interface), so the source IP may not have a route back to the incoming interface. This causes legitimate traffic to be dropped. The edge case is that uRPF strict mode does not account for asymmetric routing, while loose mode does.

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?

Read the scenario before looking for a memorised answer.

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