Question 970 of 2,152

IPv6 First Hop Security →mediumMultiple ChoiceObjective-mapped

Private VLAN IPv6 Isolation: Troubleshooting Inter-Host Communication

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.

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.

Full breakdown with real-world context →

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.”

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) on VLAN 20 isolates ports within the same VLAN, preventing direct communication between hosts on isolated ports. Even though the hosts share the same VLAN, PVLAN restricts traffic so that isolated ports can only communicate with a promiscuous port (e.g., a router uplink), not with each other. This directly explains why IPv6 hosts on VLAN 20 cannot ping each other.

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.

✓
The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication.
Why this is correct
Correct because PVLAN isolates traffic between hosts on isolated ports within the same VLAN.
Clue confirmation
The clue words "first", "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

Cisco often tests the misconception that IPv6 Source Guard or RA Guard blocks all inter-host traffic, when in fact Private VLAN is the feature specifically designed to isolate hosts within the same VLAN at Layer 2.

Detailed technical explanation

How to think about this question

Private VLANs (RFC 5517) partition a VLAN into isolated and community subdomains; isolated ports can only forward traffic to a promiscuous port (e.g., a router or firewall), effectively creating a Layer 2 barrier. In IPv6 environments, this is often combined with other First Hop Security features like RA Guard and DHCPv6 Guard to secure the access layer, but PVLAN itself is the direct cause of the inter-host communication failure. The switch's CAM table still learns MAC addresses, but the PVLAN logic drops frames between isolated ports at the hardware level.

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.

Visual reference

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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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: The switch has Private VLAN configured on VLAN 20, and the hosts are on isolated ports, which prevents direct communication. — Private VLAN (PVLAN) on VLAN 20 isolates ports within the same VLAN, preventing direct communication between hosts on isolated ports. Even though the hosts share the same VLAN, PVLAN restricts traffic so that isolated ports can only communicate with a promiscuous port (e.g., a router uplink), not with each other. This directly explains why IPv6 hosts on VLAN 20 cannot ping each other.

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.

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?

Read the scenario before looking for a memorised answer.

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