- A
The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication.
Correct because PVLAN isolates traffic between hosts on isolated ports within the same VLAN.
- B
IPv6 Source Guard is blocking inter-host traffic because the hosts' bindings are not in the binding table.
Why wrong: Incorrect because IPv6 Source Guard filters traffic based on source address to prevent spoofing, but it does not block all inter-host traffic; if both hosts have valid bindings, traffic should be allowed.
- C
RA Guard is blocking Neighbor Advertisements between hosts.
Why wrong: Incorrect because RA Guard only filters Router Advertisements and Redirects, not Neighbor Advertisements between hosts.
- D
DHCPv6 Guard is blocking DHCPv6 messages between hosts.
Why wrong: Incorrect because DHCPv6 Guard filters DHCPv6 server messages, not client-to-client traffic.
Quick Answer
The answer is that Private VLAN (PVLAN) isolation is the most likely cause, as the switch has configured isolated ports on VLAN 20, which inherently blocks all direct inter-host communication. This occurs because Private VLANs create a secondary layer of segmentation within a single broadcast domain: isolated ports can only communicate with a promiscuous port (typically a gateway), not with other isolated or community ports. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 First Hop Security features like PVLAN interact with IPv6 Source Guard—a common trap is to blame Source Guard for dropping traffic when the real issue is the port-based isolation enforced by the PVLAN configuration. Remember that even with correct IPv6 addressing and Source Guard bindings, hosts on isolated PVLAN ports will never see each other’s traffic. Memory tip: “Isolated means alone—no host-to-host, only host-to-gateway.”
300-410 IPv6 First Hop Security Practice Question
This 300-410 practice question tests your understanding of ipv6 first hop security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 is troubleshooting a network where IPv6 hosts on VLAN 20 are unable to communicate with each other. The switch is configured with IPv6 First Hop Security features including Private VLAN (PVLAN) and IPv6 Source Guard. The hosts are in the same VLAN but cannot ping each other. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"first"Why it matters: Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
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
The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication.
Private VLAN (PVLAN) isolates ports within the same VLAN, preventing communication between hosts unless they are in the same community or are promiscuous ports. If the switch has PVLAN configured, hosts on isolated ports cannot communicate directly.
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.
- ✓
The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication.
- ✗
IPv6 Source Guard is blocking inter-host traffic because the hosts' bindings are not in the binding table.
Why it's wrong here
Incorrect because IPv6 Source Guard filters traffic based on source address to prevent spoofing, but it does not block all inter-host traffic; if both hosts have valid bindings, traffic should be allowed.
- ✗
RA Guard is blocking Neighbor Advertisements between hosts.
Why it's wrong here
Incorrect because RA Guard only filters Router Advertisements and Redirects, not Neighbor Advertisements between hosts.
- ✗
DHCPv6 Guard is blocking DHCPv6 messages between hosts.
Why it's wrong here
Incorrect because DHCPv6 Guard filters DHCPv6 server messages, not client-to-client traffic.
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.
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.
What to study next
Got this wrong? Here's your next step.
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.
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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 — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication. — Private VLAN (PVLAN) isolates ports within the same VLAN, preventing communication between hosts unless they are in the same community or are promiscuous ports. If the switch has PVLAN configured, hosts on isolated ports cannot communicate directly.
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.
Are there clue words in this question I should notice?
Yes — watch for: "first", "most likely". Order matters here. You are being tested on which action comes before the others — not which action is generally useful.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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