Question 5 of 2,152

IPv6 First Hop Security →hardMultiple SelectObjective-mapped

Resolving SLAAC Issues Caused by RA Guard and ND Inspection | Cisco CCNP ENARSI 300-410

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 IPv6 connectivity issues on a switch that has IPv6 First Hop Security features enabled. Clients are unable to obtain a valid IPv6 address via SLAAC. Which TWO configuration changes could resolve this issue? (Choose TWO.)

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.

A
Modify the RA Guard policy to permit Router Advertisements from the trusted router port.
Correct. If RA Guard is blocking legitimate RAs, permitting the trusted port resolves the issue.
B
Disable DHCPv6 snooping on the VLAN to allow RAs to be forwarded.
Why wrong: Incorrect. DHCPv6 snooping does not affect RA forwarding; it only monitors DHCPv6 messages.
C
Enable IPv6 Source Guard on the client-facing interfaces.
Why wrong: Incorrect. Source Guard filters source addresses, not RAs, and would not help with RA delivery.
D
Add the router's MAC address to the ND inspection allowed-list.
Correct. ND inspection can drop RAs from unknown sources; adding the router's MAC allows them.
E
Configure a DHCPv6 pool on the switch to provide addresses to clients.
Why wrong: Incorrect. SLAAC does not use DHCPv6; a DHCPv6 pool is for stateful DHCPv6, not SLAAC.

Full breakdown with real-world context →

Quick Answer

The answer is to add the router’s MAC address to the ND inspection allowed-list and adjust the RA Guard policy to permit the router’s port. SLAAC depends on legitimate Router Advertisements (RAs) from a trusted router; RA Guard blocks RAs from unauthorized ports, while ND inspection drops RAs from unknown MAC addresses, so both features can inadvertently prevent clients from obtaining a valid IPv6 address. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of IPv6 First Hop Security features and how they interact with SLAAC—a common trap is confusing DHCPv6 snooping or Source Guard with RA filtering. Remember that SLAAC uses only RAs, not DHCPv6, so disabling DHCPv6 snooping or configuring a DHCPv6 pool is irrelevant. Memory tip: “RA Guard guards the port, ND inspects the MAC—both must trust the router for SLAAC to work.”

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

✓

Modify the RA Guard policy to permit Router Advertisements from the trusted router port.

Option A is correct because RA Guard is an IPv6 First Hop Security feature that filters Router Advertisements (RAs) based on port trust. By default, RA Guard blocks RAs on untrusted ports. Modifying the RA Guard policy to permit RAs from the trusted router port allows legitimate RAs to reach clients, enabling SLAAC address assignment.

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.

✓
Modify the RA Guard policy to permit Router Advertisements from the trusted router port.
Why this is correct
Correct. If RA Guard is blocking legitimate RAs, permitting the trusted port resolves the issue.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Disable DHCPv6 snooping on the VLAN to allow RAs to be forwarded.
Why it's wrong here
Incorrect. DHCPv6 snooping does not affect RA forwarding; it only monitors DHCPv6 messages.
✗
Enable IPv6 Source Guard on the client-facing interfaces.
Why it's wrong here
Incorrect. Source Guard filters source addresses, not RAs, and would not help with RA delivery.
✓
Add the router's MAC address to the ND inspection allowed-list.
Why this is correct
Correct. ND inspection can drop RAs from unknown sources; adding the router's MAC allows them.
Clue confirmation
The clue word "first" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure a DHCPv6 pool on the switch to provide addresses to clients.
Why it's wrong here
Incorrect. SLAAC does not use DHCPv6; a DHCPv6 pool is for stateful DHCPv6, not SLAAC.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between RA Guard (which filters RAs) and ND Inspection (which validates ND messages), leading candidates to overlook that both features can block RAs, but they operate at different layers and require different fixes.

Detailed technical explanation

How to think about this question

RA Guard operates by inspecting ICMPv6 Type 134 (Router Advertisement) messages and applying a policy (e.g., 'deny' or 'permit') based on the port role. In Cisco IOS, the 'ipv6 nd raguard' command associates a policy with a port; the default policy blocks RAs on host ports. ND Inspection (Option D) builds a binding table from Neighbor Discovery messages; adding the router's MAC address to the allowed-list ensures that RAs from that router are not dropped by ND Inspection's validation checks, which is another common cause of SLAAC failure.

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 help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.

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: Modify the RA Guard policy to permit Router Advertisements from the trusted router port. — Option A is correct because RA Guard is an IPv6 First Hop Security feature that filters Router Advertisements (RAs) based on port trust. By default, RA Guard blocks RAs on untrusted ports. Modifying the RA Guard policy to permit RAs from the trusted router port allows legitimate RAs to reach clients, enabling SLAAC address assignment.

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". 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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