- A
Loose mode (any)
Why wrong: Loose mode requires the 'any' keyword; the default is strict.
- B
Strict mode (rx)
Without keywords, the router uses strict uRPF, checking the source is reachable via the receiving interface.
- C
No uRPF is applied
Why wrong: The command itself enables uRPF; the default mode is strict.
- D
Only default routes are allowed
Why wrong: That would be 'allow-default', which is not the default.
Question 549 of 2,152
Default uRPF Mode is Strict When No Keyword Specified
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.
What is the default uRPF mode when 'ipv6 verify unicast source reachable-via' is configured without any keyword?
Quick Answer
The answer is strict mode (rx). When you configure the command 'ipv6 verify unicast source reachable-via' without any keyword, the router defaults to strict mode, meaning it checks that the source address of an incoming IPv6 packet is reachable via the exact interface on which the packet arrived. This is the most secure uRPF behavior because it drops any traffic whose return path does not match the ingress interface, preventing spoofed packets from being forwarded. On the Cisco CCNP ENARSI 300-410 exam, this default behavior is a common trap—candidates often assume they must specify a keyword, but the router applies strict mode automatically. The exam tests your understanding that omitting the keyword does not leave the feature disabled; it activates the strictest verification. A reliable memory tip is to think of the word “strict” as the default stance: if you do not explicitly choose a looser option like 'any', the router enforces the tightest possible check on the receiving interface.
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
✓
Strict mode (rx)
When 'ipv6 verify unicast source reachable-via' is configured without any keyword, the default mode is strict (rx). In strict mode, the router checks that the source address of an incoming IPv6 packet is reachable via the exact interface on which the packet was received, using the FIB. This prevents source address spoofing by ensuring the return path matches the ingress interface.
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.
- ✗
Loose mode (any)
Why it's wrong here
Loose mode requires the 'any' keyword; the default is strict.
- ✓
Strict mode (rx)
Why this is correct
Without keywords, the router uses strict uRPF, checking the source is reachable via the receiving interface.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
No uRPF is applied
Why it's wrong here
The command itself enables uRPF; the default mode is strict.
- ✗
Only default routes are allowed
Why it's wrong here
That would be 'allow-default', which is not the default.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the default behavior of commands without keywords, and the trap here is that candidates mistakenly assume 'ipv6 verify unicast source reachable-via' defaults to loose mode or requires an explicit keyword to enable uRPF, when in fact strict mode is the default.
Trap categories for this question
Keyword trap
Loose mode requires the 'any' keyword; the default is strict.
Command / output trap
The command itself enables uRPF; the default mode is strict.
Detailed technical explanation
How to think about this question
Under the hood, strict uRPF performs a reverse path lookup in the FIB for each incoming packet's source address. If the best matching route points to a different interface than the ingress interface, the packet is dropped. This behavior is defined in RFC 3704 and is critical for mitigating spoofed traffic in IPv6 networks, especially in BGP/MPLS environments where asymmetric routing is common.
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.
What to study next
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IPv6 Traffic Filtering and uRPF — study guide chapter
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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: Strict mode (rx) — When 'ipv6 verify unicast source reachable-via' is configured without any keyword, the default mode is strict (rx). In strict mode, the router checks that the source address of an incoming IPv6 packet is reachable via the exact interface on which the packet was received, using the FIB. This prevents source address spoofing by ensuring the return path matches the ingress interface.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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