- A
0 milliseconds (unspecified)
Correct. The default reachable time in the 'ipv6 nd reachable-time' command is 0, meaning no override is sent in RAs.
- B
30,000 milliseconds
Why wrong: 30,000 milliseconds is the default value used by hosts if not specified, but the Cisco default for the command is 0.
- C
60,000 milliseconds
Why wrong: 60,000 milliseconds is not the default; it is a possible manual setting.
- D
10,000 milliseconds
Why wrong: 10,000 milliseconds is not the default; it is a common manual setting.
Question 1,738 of 2,152
300-410 IPv6 First Hop Security Practice Question
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 'reachable time' in IPv6 Neighbor Discovery (ND) on Cisco IOS-XE?
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
✓
0 milliseconds (unspecified)
In IPv6 Neighbor Discovery (ND) on Cisco IOS-XE, the default value for the 'reachable time' is 0 milliseconds (unspecified). This means the router does not advertise a specific reachable time in its Router Advertisements (RAs), leaving the receiving hosts to use their own default value (typically 30,000 milliseconds as per RFC 4861). The 'reachable time' is the duration a node considers a neighbor reachable after confirming reachability via Neighbor Solicitation (NS) or Neighbor Advertisement (NA) messages.
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.
- ✓
0 milliseconds (unspecified)
Why this is correct
Correct. The default reachable time in the 'ipv6 nd reachable-time' command is 0, meaning no override is sent in RAs.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
30,000 milliseconds
Why it's wrong here
30,000 milliseconds is the default value used by hosts if not specified, but the Cisco default for the command is 0.
- ✗
60,000 milliseconds
Why it's wrong here
60,000 milliseconds is not the default; it is a possible manual setting.
- ✗
10,000 milliseconds
Why it's wrong here
10,000 milliseconds is not the default; it is a common manual setting.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the default value advertised by the router (0, meaning unspecified) and the default value used by hosts (30,000 ms), causing candidates to mistakenly select the host default as the router's advertised value.
Trap categories for this question
Command / output trap
30,000 milliseconds is the default value used by hosts if not specified, but the Cisco default for the command is 0.
Detailed technical explanation
How to think about this question
The reachable time is a field in Router Advertisement (RA) messages (ICMPv6 type 134) that informs hosts how long to consider a neighbor reachable after receiving a reachability confirmation. When set to 0, hosts apply their own default (30 seconds) as per RFC 4861 Section 6.3.2. This design allows flexibility: network administrators can override the host default by configuring a non-zero value (e.g., 'ipv6 nd reachable-time 60000') to influence ND caching behavior, which is critical in large-scale networks to reduce NS/NA overhead or prevent stale entries.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
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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: 0 milliseconds (unspecified) — In IPv6 Neighbor Discovery (ND) on Cisco IOS-XE, the default value for the 'reachable time' is 0 milliseconds (unspecified). This means the router does not advertise a specific reachable time in its Router Advertisements (RAs), leaving the receiving hosts to use their own default value (typically 30,000 milliseconds as per RFC 4861). The 'reachable time' is the duration a node considers a neighbor reachable after confirming reachability via Neighbor Solicitation (NS) or Neighbor Advertisement (NA) messages.
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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