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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
0 milliseconds (unspecified)
The default reachable time in ND is 30,000 milliseconds (30 seconds) as per RFC 4861, but Cisco IOS-XE uses a default of 0 (unspecified) in RA messages, meaning hosts use their own default (typically 30 seconds). However, the default for the 'ipv6 nd reachable-time' command is 0, indicating no override.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
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: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
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
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: 0 milliseconds (unspecified) — The default reachable time in ND is 30,000 milliseconds (30 seconds) as per RFC 4861, but Cisco IOS-XE uses a default of 0 (unspecified) in RA messages, meaning hosts use their own default (typically 30 seconds). However, the default for the 'ipv6 nd reachable-time' command is 0, indicating no override.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Question Discussion
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