Question 41 of 2,152

Device Management →hardMultiple ChoiceObjective-mapped

Unicast RPF Strict Mode Drops Legitimate Traffic Due to Asymmetric Routing

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.

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.

Full breakdown with real-world context →

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.”

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 verifies that the source IP of an incoming packet is reachable via the exact interface on which the packet arrived. In asymmetric routing, the return path to the source may use a different interface, causing the router to see the incoming interface as not matching the best return path in the FIB. This mismatch triggers a drop, even though the source IP is legitimate and reachable.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates assume any uRPF drop means the source is unreachable, when in fact asymmetric routing causes strict mode to drop legitimate traffic that would pass in loose mode.

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

Under the hood, strict uRPF performs a FIB lookup on the source IP and checks that the outgoing interface for the best prefix matches the ingress interface. In asymmetric routing, traffic may arrive on interface A while the return path uses interface B, causing a mismatch. This behavior is defined in RFC 3704, which recommends loose mode for asymmetric topologies. A real-world scenario is a dual-homed ISP connection where inbound traffic from a customer arrives on one link but the router's best route to the customer's source subnet points out the other link.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

Real-world example

How this comes up in practice

A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

Quick reference

Asymmetric Encryption Algorithm Comparison

Algorithm	Key Exchange	Signatures	Equivalent Security Key	Notes
RSA-3072	Yes	Yes	128-bit	Widely deployed; slow for bulk data
ECDSA P-256	No	Yes	128-bit	Fast signatures; standard TLS certs
ECDH / ECDHE	Yes	No	128-bit	Perfect forward secrecy in TLS 1.3
DH / DHE	Yes	No	128-bit (3072-bit key)	Replaced by ECDHE in modern TLS
Ed25519	No	Yes	~128-bit	SSH keys, modern PKI

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

Device Management — This question tests Device Management — Read the scenario before looking for a memorised answer..

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 verifies that the source IP of an incoming packet is reachable via the exact interface on which the packet arrived. In asymmetric routing, the return path to the source may use a different interface, causing the router to see the incoming interface as not matching the best return path in the FIB. This mismatch triggers a drop, even though the source IP is legitimate and reachable.

What should I do if I get this 300-410 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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