hardMultiple Choice
300-410 Practice Question: Is troubleshooting an issue where IPv6 hosts are…
A network engineer is troubleshooting an issue where IPv6 hosts are unable to perform Duplicate Address Detection (DAD) successfully. The switch is configured with IPv6 First Hop Security features including ND Inspection and ND Suppress. The engineer notices that Neighbor Solicitation messages for DAD are being dropped by the switch. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between ND Inspection and IPv6 Source Guard, where candidates mistakenly think Source Guard drops DAD messages, but it is actually ND Inspection that drops Neighbor Solicitations with an unspecified source address due to missing bindings.
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
✓
ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address.
ND Inspection drops Neighbor Solicitations with an unspecified source address (::) because it requires a valid binding for the source address in its binding table. During Duplicate Address Detection (DAD), the source address is :: (RFC 4862), which has no corresponding binding, causing ND Inspection to drop the message and preventing DAD from completing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
ND Inspection is configured to drop Neighbor Solicitations with an unspecified source address (::) because it has no binding for that address.
Why this is correct
During Duplicate Address Detection, a host sends Neighbor Solicitations from the unspecified address (::) before it owns any address. ND Inspection requires a binding between source address and link-layer address; with no binding for ::, it drops these messages, breaking DAD.
- ✗
RA Guard is configured to drop all multicast traffic, including Neighbor Solicitations.
Why it's wrong here
RA Guard inspects Router Advertisement and redirect messages only; it does not drop multicast Neighbor Solicitations, and no RA Guard mode blocks all multicast. It is tempting because RA Guard is a First Hop Security feature policing ICMPv6, but its filter scope is RAs, not NS messages used for DAD.
- ✗
DHCPv6 Guard is blocking the DAD messages because they are considered DHCPv6 traffic.
Why it's wrong here
DHCPv6 Guard filters DHCPv6 server and client messages on UDP ports 546 and 547; DAD Neighbor Solicitations are ICMPv6 type 135 and never match its policies. It is tempting because both features police IPv6 control traffic on the same switch ports, but DHCPv6 Guard's message-type scope excludes ND traffic entirely.
- ✗
IPv6 Source Guard is dropping the DAD messages because the source address :: is not in the binding table.
Why it's wrong here
IPv6 Source Guard permits unspecified-address (::) sources only for DAD Neighbor Solicitations; it does not drop them for lacking a binding-table entry, so it cannot be the cause here. It is tempting because Source Guard does filter on binding-table state, but ND Inspection, not Source Guard, drops DAD messages when the target address is absent from its binding table.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.