This 300-410 practice question tests your understanding of ipv6 first hop security. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
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.
The 'allow-default' option in 'ipv6 verify source' allows traffic with a source address that matches the default route (::/0) in the binding table. However, this option only works if the binding table has an entry for the default prefix (::/0). If the router's traffic is being dropped, it might be because the router is using a global unicast source address that is not the default route. The edge case is that 'allow-default' is often misunderstood: it does not allow all traffic; it only allows traffic whose source address matches a binding entry for the default route. If the router's source address is a specific global address, that address must be in the binding table individually. The engineer likely thought 'allow-default' would permit all traffic, but it only permits traffic from the default prefix.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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
Standard ACLs match source addresses.
✗
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: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — Standard ACLs match source addresses..
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. — The 'allow-default' option in 'ipv6 verify source' allows traffic with a source address that matches the default route (::/0) in the binding table. However, this option only works if the binding table has an entry for the default prefix (::/0). If the router's traffic is being dropped, it might be because the router is using a global unicast source address that is not the default route. The edge case is that 'allow-default' is often misunderstood: it does not allow all traffic; it only allows traffic whose source address matches a binding entry for the default route. If the router's source address is a specific global address, that address must be in the binding table individually. The engineer likely thought 'allow-default' would permit all traffic, but it only permits traffic from the default prefix.
What should I do if I get this 300-410 question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related 300-410 ACL questions on filtering logic and placement.
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?
Standard ACLs match source addresses.
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