The answer is that IPv6 Source Guard drops OSPFv3 packets because it does not learn link-local addresses via ND snooping, leaving the binding table empty for those addresses. IPv6 Snooping and Source Guard rely on Neighbor Discovery snooping to populate the binding table, but OSPFv3 communicates exclusively using link-local addresses, which are algorithmically derived from the interface MAC and are never advertised in Neighbor Advertisements. Since the switch never sees a valid ND message for the router’s link-local address, no binding entry is created, and Source Guard drops the traffic as unauthorized. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of the IPv6 First Hop Security feature set and its edge cases—specifically that link-local bindings are not automatically generated, causing routing protocol failures. A common trap is assuming ND snooping captures all IPv6 addresses, but OSPFv3’s use of link-local sources is a deliberate blind spot. Memory tip: “Link-local is local to the link, not learned by ND—so Source Guard sinks the OSPF.”
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.
A network engineer configures 'ipv6 snooping' globally on a switch and applies 'ipv6 verify source' on a port connected to a router running OSPFv3. The router's OSPFv3 neighborship with another router across the switch fails. The switch logs show that OSPFv3 packets are being dropped. The engineer checks the binding table and sees no entries for the router's link-local address. What is the most likely reason?
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
✓
Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped.
IPv6 Snooping and Source Guard typically rely on ND snooping to populate the binding table. However, OSPFv3 uses link-local addresses for communication, and link-local addresses are not learned via ND snooping because they are derived from the interface MAC address and are not advertised in NAs. The switch does not create binding entries for link-local addresses unless explicitly configured. Therefore, Source Guard drops OSPFv3 packets because the source link-local address is not in the binding table. The edge case is that IPv6 First Hop Security features often overlook link-local addresses, causing routing protocol failures.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped.
Why this is correct
Link-local addresses are not populated in the binding table by default.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
OSPFv3 uses multicast addresses that are blocked by Source Guard.
Why it's wrong here
Source Guard validates source address, not destination.
✗
The router must be configured as a static binding for its link-local address.
Why it's wrong here
Static binding is an option but not required; the issue is the lack of automatic learning.
✗
The switch must have 'ipv6 snooping' enabled on the VLAN, not globally.
Why it's wrong here
Global enables it for all VLANs.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
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 OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
IPv6 First Hop Security — This question tests IPv6 First Hop Security — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped. — IPv6 Snooping and Source Guard typically rely on ND snooping to populate the binding table. However, OSPFv3 uses link-local addresses for communication, and link-local addresses are not learned via ND snooping because they are derived from the interface MAC address and are not advertised in NAs. The switch does not create binding entries for link-local addresses unless explicitly configured. Therefore, Source Guard drops OSPFv3 packets because the source link-local address is not in the binding table. The edge case is that IPv6 First Hop Security features often overlook link-local addresses, causing routing protocol failures.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
About these practice questions
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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. A network engineer is troubleshooting an issue where IPv6 traffic is being forwarded incorrectly on a switch. The switch is configured with IPv6 Source Guard on access ports. A legitimate host on port Fa0/1 with IPv6 address 2001:db8:1::10 is unable to send traffic to the default gateway. The engineer checks the IPv6 binding table and sees that the host's entry is missing. What is the most likely cause?
medium
✓ A.The host is using a static IPv6 address, and ND snooping is not enabled on the VLAN, so the binding was never learned.
B.The host's MAC address is not in the MAC address table for VLAN 1.
C.The switch is running IPv6 First Hop Security in monitor mode, which logs violations but does not drop traffic.
D.The default gateway router is not sending Router Advertisements, so the host cannot form a default route.
Why A: IPv6 Source Guard requires a valid binding entry (learned via DHCPv6 snooping or ND snooping) to permit traffic. If the host is using a static IPv6 address, ND snooping must be enabled to learn the binding; otherwise, traffic is dropped.
Last reviewed: Jun 18, 2026
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