- A
The router receives the packet on an interface that is not the best return path to the source IP, causing strict uRPF to drop it.
Strict uRPF requires the incoming interface to be the same as the outgoing interface for the source IP; asymmetric routing violates this.
- B
The source IP is not in the routing table at all.
Why wrong: If the source IP is not in the routing table, uRPF would drop it, but the scenario says the source is legitimate.
- C
The uRPF configuration is missing the 'allow-default' option.
Why wrong: The 'allow-default' option allows the use of a default route for the reverse path check, but it does not fix asymmetric routing issues.
- D
The router is using loose mode instead of strict mode.
Why wrong: Loose mode only checks that the source IP is in the routing table, not the interface, so it would not drop the traffic.
Quick Answer
The answer is that strict uRPF drops the traffic because the router receives the packet on an interface that is not the best return path to the source IP. This happens because strict mode performs a reverse path lookup, verifying that the incoming interface matches the optimal outgoing interface for the source address in the routing table. In asymmetric routing, packets from a legitimate source may arrive via a different path than the router would use to reply, causing the strict check to fail and the traffic to be dropped. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of how uRPF interacts with routing asymmetries, often appearing as a trick question where candidates mistakenly blame a routing loop or ACL. A common trap is assuming strict mode works like loose mode, which only checks for a route existence. Remember the memory tip: “Strict means stick to the same interface; if the path splits, the packet quits.”
300-410 Device Management Practice Question
This 300-410 practice question tests your understanding of device management. 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 unicast Reverse Path Forwarding (uRPF) in strict mode on an interface. Traffic from a legitimate source IP is being dropped. The network has asymmetric routing. 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 router receives the packet on an interface that is not the best return path to the source IP, causing strict uRPF to drop it.
Strict uRPF checks that the source IP of incoming traffic matches the best reverse route via the same interface. In asymmetric routing, traffic may arrive on an interface different from the one the router would use to send traffic back to that source, causing strict uRPF to drop the traffic.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 router receives the packet on an interface that is not the best return path to the source IP, causing strict uRPF to drop it.
Why this is correct
Strict uRPF requires the incoming interface to be the same as the outgoing interface for the source IP; asymmetric routing violates this.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The source IP is not in the routing table at all.
Why it's wrong here
If the source IP is not in the routing table, uRPF would drop it, but the scenario says the source is legitimate.
- ✗
The uRPF configuration is missing the 'allow-default' option.
Why it's wrong here
The 'allow-default' option allows the use of a default route for the reverse path check, but it does not fix asymmetric routing issues.
- ✗
The router is using loose mode instead of strict mode.
Why it's wrong here
Loose mode only checks that the source IP is in the routing table, not the interface, so it would not drop the traffic.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Trap categories for this question
Scenario analysis trap
If the source IP is not in the routing table, uRPF would drop it, but the scenario says the source is legitimate.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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Device Management — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Management — This question tests Device Management — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The router receives the packet on an interface that is not the best return path to the source IP, causing strict uRPF to drop it. — Strict uRPF checks that the source IP of incoming traffic matches the best reverse route via the same interface. In asymmetric routing, traffic may arrive on an interface different from the one the router would use to send traffic back to that source, causing strict uRPF to drop the traffic.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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