Question 1,526 of 2,152

IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

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 configures 'ipv6 verify source' with 'allow-default' on a switch port connected to a router that uses a default route via a static route. The router's traffic is being dropped by Source Guard. The engineer sees that the router's source address is in the binding table. 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: "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 'allow-default' option only permits traffic with source address matching the default route entry, not all traffic.
It allows traffic from the default prefix, not all sources.
B
The router's source address is a link-local address, which is not supported by Source Guard.
Why wrong: Link-local addresses are supported but must be in the binding table.
C
The 'allow-default' option requires the router to send an NA for the default route.
Why wrong: The binding entry for default must be configured statically.
D
The switch port must be configured as 'trusted' for Source Guard to work with routers.
Why wrong: No trust concept for Source Guard.

Full breakdown with real-world context →

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 'allow-default' option only permits traffic with source address matching the default route entry, not all traffic.

Option A is correct because the 'allow-default' keyword in the 'ipv6 verify source' command only permits traffic whose source IPv6 address matches the default route entry (::/0) in the binding table. It does not allow all traffic; it specifically allows traffic from the default router's link-local address that is associated with the default route. Since the router's source address is in the binding table but the traffic is still dropped, the issue is that the router is sending packets with a source address that does not match the default route entry, or the default route entry itself is not properly installed in the binding table. The 'allow-default' option is not a blanket pass for all traffic from the router.

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 'allow-default' option only permits traffic with source address matching the default route entry, not all traffic.
Why this is correct
It allows traffic from the default prefix, not all sources.
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 router's source address is a link-local address, which is not supported by Source Guard.
Why it's wrong here
Link-local addresses are supported but must be in the binding table.
✗
The 'allow-default' option requires the router to send an NA for the default route.
Why it's wrong here
The binding entry for default must be configured statically.
✗
The switch port must be configured as 'trusted' for Source Guard to work with routers.
Why it's wrong here
No trust concept for Source Guard.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that 'allow-default' permits all traffic from a router, when in fact it only permits traffic matching the specific default route entry in the binding table, typically the router's link-local address learned via RA.

Detailed technical explanation

How to think about this question

The 'ipv6 verify source' command with 'allow-default' leverages the IPv6 First Hop Security binding table, which is populated by ND snooping and DHCPv6 snooping. The default route entry in the binding table is created when a router sends a Router Advertisement (RA) with a non-zero Router Lifetime, and the switch records the router's link-local address as the default router. The 'allow-default' option permits traffic from that specific link-local source address, but only if the destination is the all-nodes multicast address (FF02::1) or the traffic is part of the default route's next-hop validation. In real-world scenarios, if the router uses a static default route pointing to a next-hop that is not the switch's learned default router, the 'allow-default' option will not match, and traffic will be dropped.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

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 'allow-default' option only permits traffic with source address matching the default route entry, not all traffic. — Option A is correct because the 'allow-default' keyword in the 'ipv6 verify source' command only permits traffic whose source IPv6 address matches the default route entry (::/0) in the binding table. It does not allow all traffic; it specifically allows traffic from the default router's link-local address that is associated with the default route. Since the router's source address is in the binding table but the traffic is still dropped, the issue is that the router is sending packets with a source address that does not match the default route entry, or the default route entry itself is not properly installed in the binding table. The 'allow-default' option is not a blanket pass for all traffic from the router.

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.

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