Question 475 of 2,152

IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

IPv6 Source Guard Static Binding — Why ND Entry is Required | 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 configures IPv6 Source Guard on a switch port with 'ipv6 verify source' and also enables 'ipv6 snooping' globally. A legitimate host on that port is unable to send traffic, and the switch logs show that packets are being dropped due to source address validation failure. The host has a static IPv6 address and the engineer has configured a static binding using 'ipv6 neighbor binding' command. What is the most likely oversight?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

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 static binding must be associated with a valid ND entry; without an NA from the host, the binding remains incomplete.
Source Guard requires the binding to be in REACHABLE state, which requires ND activity.
B
The 'ipv6 verify source' command must include the 'allow-default' option to work with static addresses.
Why wrong: allow-default is for uRPF, not Source Guard.
C
The switch port must be configured as 'trusted' for IPv6 snooping.
Why wrong: Trust is for DHCP Guard, not Source Guard.
D
The host must use DHCPv6 to obtain an address for Source Guard to work.
Why wrong: Source Guard works with static addresses if bindings are present.

Full breakdown with real-world context →

Quick Answer

The answer is that the static binding must be associated with a valid ND entry; without an NA from the host, the binding remains in an INCOMPLETE state. IPv6 Source Guard with `ipv6 verify source` relies on the binding table to validate source addresses, and even a manually configured `ipv6 neighbor binding` entry will not be used for forwarding unless the switch has a corresponding ND cache entry in the REACHABLE state. This is because the binding table entry is tied to the ND process—if the host never sends an unsolicited Neighbor Advertisement or the switch never receives a Router Solicitation to populate the ND cache, the binding stays incomplete and Source Guard drops the traffic. On the CCNP ENARSI 300-410 exam, this tests your understanding that static bindings do not automatically create ND entries; it is a common trap to assume a static binding alone is sufficient. Remember the memory tip: “Static binding needs an NA to play—no NA, no REACHABLE, no traffic.”

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 static binding must be associated with a valid ND entry; without an NA from the host, the binding remains incomplete.

The correct answer is A because IPv6 Source Guard relies on the Neighbor Discovery (ND) cache to validate source addresses. Even with a static binding configured via 'ipv6 neighbor binding', the binding remains in an 'incomplete' state until the switch receives a valid Neighbor Advertisement (NA) from the host. Without a complete ND entry, the switch drops packets from the host as the source address cannot be verified against a valid binding.

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 static binding must be associated with a valid ND entry; without an NA from the host, the binding remains incomplete.
Why this is correct
Source Guard requires the binding to be in REACHABLE state, which requires ND activity.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The 'ipv6 verify source' command must include the 'allow-default' option to work with static addresses.
Why it's wrong here
allow-default is for uRPF, not Source Guard.
✗
The switch port must be configured as 'trusted' for IPv6 snooping.
Why it's wrong here
Trust is for DHCP Guard, not Source Guard.
✗
The host must use DHCPv6 to obtain an address for Source Guard to work.
Why it's wrong here
Source Guard works with static addresses if bindings are present.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that a static binding alone is sufficient for IPv6 Source Guard, when in fact the binding must be resolved via ND (NA message) to become active and allow traffic.

Detailed technical explanation

How to think about this question

IPv6 Source Guard uses the ND snooping database (built from Neighbor Solicitations and Advertisements) to validate source IPv6 addresses in data packets. A static binding configured with 'ipv6 neighbor binding' creates a binding entry, but it remains in an 'incomplete' state until the switch observes a corresponding NA from the host, which populates the ND cache with a resolved link-layer address. In real-world scenarios, if a host is powered on after the static binding is configured, it may not immediately send an NA, causing traffic drops until the host sends an unsolicited NA or the switch triggers NS-based resolution.

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.

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 static binding must be associated with a valid ND entry; without an NA from the host, the binding remains incomplete. — The correct answer is A because IPv6 Source Guard relies on the Neighbor Discovery (ND) cache to validate source addresses. Even with a static binding configured via 'ipv6 neighbor binding', the binding remains in an 'incomplete' state until the switch receives a valid Neighbor Advertisement (NA) from the host. Without a complete ND entry, the switch drops packets from the host as the source address cannot be verified against a valid binding.

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: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

About these practice questions

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Which configuration is missing to properly implement IPv6 First Hop Security on an access switch port that should only allow traffic from a single host with a static IPv6 address 2001:db8:1::10?

medium

✓ A.The interface needs 'ipv6 verify source' and a static binding entry 'ipv6 source binding 2001:db8:1::10 interface GigabitEthernet0/6'.
B.The interface needs 'ipv6 nd raguard' to block RAs.
C.The interface needs 'ipv6 dhcp guard' to block DHCP messages.
D.The interface needs 'ipv6 nd inspection' to validate ND messages.

Why A: Option A is correct because IPv6 First Hop Security (FHS) on an access switch port requires both the 'ipv6 verify source' command on the interface and a static IPv6 source binding entry to enforce that only traffic from the specified host (2001:db8:1::10) is allowed. The 'ipv6 verify source' command enables source address validation using the binding table, and the static binding entry populates that table with the allowed source address and interface. Without both components, the switch cannot filter traffic based on the static IPv6 address.

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