Question 170 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

300-410 NetFlow and Flexible NetFlow Practice Question

This 300-410 practice question tests your understanding of netflow and flexible netflow. 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. After the configuration, legitimate traffic from a customer network is being dropped. The engineer verifies that the customer's IP prefix is in the routing table. 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 router has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet.
Strict uRPF requires that the best route to the source points back to the receiving interface. If there are multiple equal-cost paths, the router may choose a different interface for the return path, causing drops.
B
The customer network uses private IP addresses that are not routable, so uRPF drops them.
Why wrong: uRPF does not care about private vs public IPs; it only checks the routing table.
C
The interface is configured with 'ip verify unicast source reachable-via any', which is loose mode, not strict mode.
Why wrong: The scenario says strict mode is configured, so this is not the issue.
D
The routing table has a default route that points to a different interface, causing uRPF to use the default route for verification.
Why wrong: By default, uRPF does not use default routes for verification unless the 'allow-default' option is configured. If a default route exists, it may cause issues, but the scenario says the customer prefix is in the routing table, so the default route is not used.

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 router has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet.

Strict uRPF checks that the source IP of incoming packets has a route back to the same interface. If there is asymmetric routing, where the return path goes out a different interface, strict uRPF will drop the packets. A common edge case is when the router has multiple equal-cost paths to the source network, and the return traffic uses a different interface than the one the packet arrived on. In such cases, uRPF strict mode will fail because it only checks the best route, not all routes.

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 has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet.
Why this is correct
Strict uRPF requires that the best route to the source points back to the receiving interface. If there are multiple equal-cost paths, the router may choose a different interface for the return path, causing drops.
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 customer network uses private IP addresses that are not routable, so uRPF drops them.
Why it's wrong here
uRPF does not care about private vs public IPs; it only checks the routing table.
✗
The interface is configured with 'ip verify unicast source reachable-via any', which is loose mode, not strict mode.
Why it's wrong here
The scenario says strict mode is configured, so this is not the issue.
✗
The routing table has a default route that points to a different interface, causing uRPF to use the default route for verification.
Why it's wrong here
By default, uRPF does not use default routes for verification unless the 'allow-default' option is configured. If a default route exists, it may cause issues, but the scenario says the customer prefix is in the routing table, so the default route is not used.

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
The scenario says strict mode is configured, so this is not the issue.

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?

NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: The router has multiple equal-cost paths to the customer network, and the return path uses a different interface, causing strict uRPF to drop the packet. — Strict uRPF checks that the source IP of incoming packets has a route back to the same interface. If there is asymmetric routing, where the return path goes out a different interface, strict uRPF will drop the packets. A common edge case is when the router has multiple equal-cost paths to the source network, and the return traffic uses a different interface than the one the packet arrived on. In such cases, uRPF strict mode will fail because it only checks the best route, not all routes.

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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