- 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.
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.”
300-410 VRF-Lite Practice Question
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.
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: 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 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
Static NAT maps one inside address to one outside address.
- ✗
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: 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.
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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VRF-Lite — study guide chapter
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VRF-Lite practice questions
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FAQ
Questions learners often ask
What does this 300-410 question test?
VRF-Lite — This question tests VRF-Lite — Static NAT maps one inside address to one outside address..
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?
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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