Question 270 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Strict Mode and ECMP Asymmetric Routing

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 uRPF strict mode on an interface that is part of an IPv6 BGP multihoming setup with two upstream providers. The router receives BGP routes from both providers and has equal-cost paths to some destinations. Traffic from the router to those destinations is intermittently 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.

A
ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check.
Correct. uRPF strict mode expects symmetric routing; ECMP breaks that assumption.
B
BGP routes are not installed in the routing table, so uRPF has no entry to check.
Why wrong: Incorrect. BGP routes are installed in the routing table if they are best paths.
C
The router has 'ipv6 cef' disabled, causing uRPF to fail.
Why wrong: Incorrect. uRPF requires CEF, but the symptom is intermittent drops, not complete failure.
D
The upstream providers are using different AS paths, causing BGP to not install equal-cost paths.
Why wrong: Incorrect. Equal-cost paths can exist even with different AS paths if the IGP metric is equal.

Full breakdown with real-world context →

Quick Answer

The correct answer is that ECMP causes asymmetric routing, which violates uRPF strict mode’s single-interface check. uRPF strict mode requires that any incoming packet’s source address be reachable via the exact interface on which the packet arrived, using the router’s FIB for the reverse path lookup. When equal-cost multipath is active, traffic from the same source can arrive on different interfaces due to load balancing, while the return path may use a different interface—this asymmetry means the packet’s arrival interface does not match the best reverse path, so uRPF drops it. On the CCNP ENARSI 300-410 exam, this scenario tests your understanding of how uRPF interacts with ECMP in multihomed IPv6 BGP setups; a common trap is assuming uRPF only checks source validity, not interface consistency. Remember the memory tip: “Strict mode is a one-interface party—ECMP brings uninvited guests on another door.”

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

✓

ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check.

With uRPF strict mode enabled, the router checks that the source address of an incoming packet matches a route in the FIB and that the incoming interface is the same as the interface used to reach that source. In an ECMP scenario with two upstream providers, traffic from the router to a destination may be load-balanced across both links, but return traffic from that destination may arrive on a different interface than the one uRPF expects. This asymmetric routing causes uRPF strict mode to drop the packets, as they fail the single-interface check.

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.

✓
ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check.
Why this is correct
Correct. uRPF strict mode expects symmetric routing; ECMP breaks that assumption.
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.
✗
BGP routes are not installed in the routing table, so uRPF has no entry to check.
Why it's wrong here
Incorrect. BGP routes are installed in the routing table if they are best paths.
✗
The router has 'ipv6 cef' disabled, causing uRPF to fail.
Why it's wrong here
Incorrect. uRPF requires CEF, but the symptom is intermittent drops, not complete failure.
✗
The upstream providers are using different AS paths, causing BGP to not install equal-cost paths.
Why it's wrong here
Incorrect. Equal-cost paths can exist even with different AS paths if the IGP metric is equal.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the subtle interaction between uRPF strict mode and ECMP, where candidates mistakenly think uRPF works with any valid route, but they overlook the requirement that the incoming interface must match the FIB's outgoing interface for the source prefix.

Detailed technical explanation

How to think about this question

uRPF strict mode performs a FIB lookup on the source IP address of each incoming packet and verifies that the packet arrived on the interface that the FIB would use to reach that source. In ECMP scenarios, the FIB may have multiple next-hop interfaces for the same prefix, but uRPF strict mode only checks against the first next-hop interface in the FIB entry. This means that if return traffic arrives on a different ECMP member interface, it is dropped, even though the route is valid. A workaround is to use uRPF loose mode or to apply policy-based routing to force symmetric paths.

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?

IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: ECMP causes asymmetric routing where packets from the same source may arrive on different interfaces, violating uRPF strict mode's single-interface check. — With uRPF strict mode enabled, the router checks that the source address of an incoming packet matches a route in the FIB and that the incoming interface is the same as the interface used to reach that source. In an ECMP scenario with two upstream providers, traffic from the router to a destination may be load-balanced across both links, but return traffic from that destination may arrive on a different interface than the one uRPF expects. This asymmetric routing causes uRPF strict mode to drop the packets, as they fail the single-interface check.

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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