- 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.
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.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
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.
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.
uRPF strict mode requires that the source address of incoming packets be reachable via the same interface on which the packet arrived. With equal-cost multipath (ECMP), traffic may arrive on a different interface than the one used for the return path (asymmetric routing). uRPF strict mode drops packets that arrive on an interface that is not the best reverse path. This is a classic edge case: ECMP can cause asymmetric flows, breaking uRPF strict mode.
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.
- ✓
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
Static NAT maps one inside address to one outside address.
- ✗
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: 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?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Static NAT maps one inside address to one outside address..
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. — uRPF strict mode requires that the source address of incoming packets be reachable via the same interface on which the packet arrived. With equal-cost multipath (ECMP), traffic may arrive on a different interface than the one used for the return path (asymmetric routing). uRPF strict mode drops packets that arrive on an interface that is not the best reverse path. This is a classic edge case: ECMP can cause asymmetric flows, breaking uRPF strict mode.
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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Last reviewed: Jun 18, 2026
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