- A
8
Why wrong: 8 is not the default; it is a common manual setting.
- B
16
Correct. The default limit is 16 prefixes.
- C
32
Why wrong: 32 is not the default; it is a possible manual setting.
- D
64
Why wrong: 64 is not the default; it is too high for typical configurations.
Question 1,111 of 2,152
Default Number of IPv6 Prefixes in RA: 16
This 300-410 practice question tests your understanding of ipv6 first hop security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 value of the 'limit' parameter in the 'ipv6 nd prefix' command for the number of prefixes advertised in RA messages?
Quick Answer
The answer is 16. This default value for the `limit` parameter in the `ipv6 nd prefix` command controls the maximum number of IPv6 prefixes that a Cisco router will include in a single Router Advertisement (RA) message, a Cisco-specific behavior not mandated by RFC 4861. On the CCNP ENARSI 300-410 exam, this tests your understanding of IPv6 Neighbor Discovery (ND) configuration and the distinction between Cisco defaults and RFC standards. A common trap is assuming the limit is infinite or RFC-defined, but Cisco IOS-XE caps it at 16 to prevent RA messages from becoming too large. To remember this, think of the number 16 as the "sweet sixteen" limit—just enough prefixes for most enterprise scenarios without overwhelming the RA packet.
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
✓
16
The default value of the 'limit' parameter in the 'ipv6 nd prefix' command is 16, as defined by Cisco IOS. This parameter controls the maximum number of IPv6 prefixes that can be advertised in Router Advertisement (RA) messages. When the number of prefixes exceeds this limit, the router will not include additional prefixes in the RA, ensuring the message stays within reasonable size constraints.
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.
- ✗
8
Why it's wrong here
8 is not the default; it is a common manual setting.
- ✓
16
Why this is correct
Correct. The default limit is 16 prefixes.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
32
Why it's wrong here
32 is not the default; it is a possible manual setting.
- ✗
64
Why it's wrong here
64 is not the default; it is too high for typical configurations.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the default value of the 'limit' parameter to catch candidates who confuse it with other common IPv6 defaults, such as the 64-bit prefix length or the 32-byte option size, leading them to pick 8, 32, or 64 instead of the correct 16.
Detailed technical explanation
How to think about this question
The 'ipv6 nd prefix' command is used to configure prefixes for Neighbor Discovery (ND) Router Advertisements, and the 'limit' parameter ensures that the RA message does not exceed the maximum transmission unit (MTU) of the link, typically 1500 bytes. Under the hood, each prefix option in an RA consumes about 32 bytes, so a limit of 16 allows up to 512 bytes for prefixes, leaving room for other options like MTU, DNS, and flags. In real-world scenarios, exceeding this limit can cause fragmentation or dropped RAs, leading to connectivity issues for hosts that rely on stateless address autoconfiguration (SLAAC).
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
Got this wrong? Here's your next step.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 First Hop Security — This question tests IPv6 First Hop Security — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 16 — The default value of the 'limit' parameter in the 'ipv6 nd prefix' command is 16, as defined by Cisco IOS. This parameter controls the maximum number of IPv6 prefixes that can be advertised in Router Advertisement (RA) messages. When the number of prefixes exceeds this limit, the router will not include additional prefixes in the RA, ensuring the message stays within reasonable size constraints.
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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