Question 978 of 2,152

Administrative Distance →hardMultiple ChoiceObjective-mapped

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."

300-410 Administrative Distance Practice Question

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.

Question 1hardmultiple choice

Read the full NAT/PAT explanation →

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.

Full breakdown with real-world context →

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: 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'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
Static NAT maps one inside address to one outside address.
✗
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: 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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FAQ

Questions learners often ask

What does this 300-410 question test?

Administrative Distance — This question tests Administrative Distance — 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'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?

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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