The answer is that strict uRPF drops legitimate traffic when asymmetric routing forces the return path to the source IP out a different interface than the one where the packet arrived. Strict mode performs a two-part check: it verifies that the source address exists in the routing table and, critically, that the best reverse path to that source exits through the exact interface where the packet was received. When traffic from a customer network enters via one interface but the router’s best route back to that source points out a different interface—a classic asymmetric routing scenario—the strict check fails, and the packet is silently dropped. On the Cisco CCNP ENARSI 300-410 exam, this is a favorite trap: candidates often assume uRPF only checks for a route, forgetting the interface-matching requirement. The key memory tip is “Strict means same interface in and out; if the path splits, the packet splits.”
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. 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 facing the Internet. Legitimate traffic from a customer network is being dropped. The traffic has a source IP that belongs to the customer's prefix, which is reachable via a different interface on the router. 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The customer traffic is arriving on an interface where the return path to the source is via a different interface, violating strict uRPF.
Strict uRPF checks that the source IP of incoming packets has a route back out the same interface. If the best path to the source IP is via a different interface (asymmetric routing), the packet is dropped. This is a classic edge case with strict uRPF in asymmetric routing scenarios.
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 'ip verify unicast source reachable-via any' command was used instead of 'rx'.
Why it's wrong here
That would be loose mode, which would not drop the traffic.
✗
The router has a default route pointing out the same interface, causing uRPF to pass all traffic.
Why it's wrong here
Default route can cause uRPF to pass traffic that should be dropped.
✓
The customer traffic is arriving on an interface where the return path to the source is via a different interface, violating strict uRPF.
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 'ip urpf allow-default' command is missing, causing default routes to be ignored.
Why it's wrong here
Allow-default affects loose mode, not strict mode.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Route Redistribution — This question tests Route Redistribution — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The customer traffic is arriving on an interface where the return path to the source is via a different interface, violating strict uRPF. — Strict uRPF checks that the source IP of incoming packets has a route back out the same interface. If the best path to the source IP is via a different interface (asymmetric routing), the packet is dropped. This is a classic edge case with strict uRPF in asymmetric routing scenarios.
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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