- A
The uRPF strict mode requires that the source IP's best path is out the same interface; asymmetric routing violates this.
Strict mode drops packets if the reverse path does not match the incoming interface, which occurs with asymmetric routing.
- B
The uRPF loose mode should be used instead, as it only requires a route to the source IP in the FIB.
Why wrong: While loose mode would work, the question asks for the explanation of the drop, not the fix.
- C
The customer network is using private IP addresses that are not routable.
Why wrong: Private IPs would be dropped at the ISP edge, but the issue is asymmetric routing.
- D
The uRPF allow-default option is missing, which is required for default routes.
Why wrong: Allow-default is for default routes, not for asymmetric routing.
uRPF Strict Mode and Asymmetric Routing
This 300-410 practice question tests your understanding of administrative distance. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 enables uRPF (strict mode) on an interface facing the Internet. Legitimate traffic from a customer network is being dropped. The customer network uses asymmetric routing where return traffic takes a different path. 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.
Quick Answer
The answer is that uRPF strict mode drops the traffic because it requires the source IP address of an incoming packet to have a route in the FIB pointing back out the exact same interface on which the packet arrived. Asymmetric routing violates this fundamental check because return traffic for that source is leaving via a different path, so the router sees the source as unreachable through the ingress interface and discards the packet. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how uRPF interacts with FIB-based reverse path lookup, and it is a common trap to assume strict mode can handle asymmetric flows—it cannot. A key memory tip is "Strict is strict: same in, same out; if the path splits, the packet gets hits."
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's best path is out the same interface; asymmetric routing violates this.
Strict uRPF checks that the source IP of incoming packets has a route in the FIB pointing back to the same interface. With asymmetric routing, the return path may use a different interface, causing the check to fail and the packet to be dropped.
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's best path is out the same interface; asymmetric routing violates this.
Why this is correct
Strict mode drops packets if the reverse path does not match the incoming interface, which occurs with asymmetric routing.
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 should be used instead, as it only requires a route to the source IP in the FIB.
Why it's wrong here
While loose mode would work, the question asks for the explanation of the drop, not the fix.
- ✗
The customer network is using private IP addresses that are not routable.
Why it's wrong here
Private IPs would be dropped at the ISP edge, but the issue is asymmetric routing.
- ✗
The uRPF allow-default option is missing, which is required for default routes.
Why it's wrong here
Allow-default is for default routes, not for asymmetric routing.
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?
Administrative Distance — This question tests Administrative Distance — 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's best path is out the same interface; asymmetric routing violates this. — Strict uRPF checks that the source IP of incoming packets has a route in the FIB pointing back to the same interface. With asymmetric routing, the return path may use a different interface, causing the check to fail and the packet to be dropped.
What should I do if I get this 300-410 question wrong?
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.
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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Last reviewed: Jun 18, 2026
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