Question 879 of 2,152

IPv6 First Hop Security →hardMultiple ChoiceObjective-mapped

Troubleshoot IPv6 Source Guard Dropping OSPFv3 Due to Missing Link-Local Binding

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.

A
Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped.
Link-local addresses are not populated in the binding table by default.
B
OSPFv3 uses multicast addresses that are blocked by Source Guard.
Why wrong: Source Guard validates source address, not destination.
C
The router must be configured as a static binding for its link-local address.
Why wrong: Static binding is an option but not required; the issue is the lack of automatic learning.
D
The switch must have 'ipv6 snooping' enabled on the VLAN, not globally.
Why wrong: Global enables it for all VLANs.

Full breakdown with real-world context →

Quick Answer

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.”

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

✓

Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped.

Option A is correct because IPv6 Source Guard, enabled by 'ipv6 verify source', relies on the IPv6 binding table to permit traffic. The binding table is populated by IPv6 Neighbor Discovery (ND) snooping, which learns global unicast and link-local addresses from ND messages. However, OSPFv3 uses its own Hello and LSA packets sent to the multicast address FF02::5, which are not ND messages. Since the router's link-local address is not learned via ND snooping, no binding entry exists, and Source Guard drops the OSPFv3 packets, causing the neighborship to fail.

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.

✓
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.
Related concept
Read the scenario before looking for a memorised 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: answer the scenario, not the keyword

Cisco often tests the misconception that IPv6 Source Guard blocks multicast traffic, when in fact it filters based on source address validity against the binding table, not the destination address type.

Detailed technical explanation

How to think about this question

IPv6 Source Guard (RFC 6621) validates the source IPv6 address of incoming packets against the binding table, which is built by ND snooping (RFC 4861) and DHCPv6 snooping. OSPFv3 (RFC 5340) uses link-local addresses for neighbor discovery and routing updates, but these addresses are not learned via ND snooping because OSPFv3 packets are not ND messages. In real-world deployments, this issue is often resolved by either disabling IPv6 Source Guard on OSPFv3-enabled ports or by configuring a static binding for the router's link-local address, though the latter is cumbersome in dynamic environments.

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.

Quick reference

Routing Protocol Comparison

Protocol	Metric	Max Hops	Algorithm	Type
RIP v2	Hop count	15	Bellman-Ford	Distance vector
OSPF	Cost (bandwidth)	Unlimited	Dijkstra (SPF)	Link state
EIGRP	Composite metric	Unlimited	DUAL	Hybrid
IS-IS	Cost	Unlimited	Dijkstra	Link state
BGP	Policy / attributes	Unlimited	Path vector	Path vector

RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.

What to study next

Got this wrong? Here's your next step.

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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: Source Guard does not learn link-local addresses via ND snooping, so OSPFv3 packets are dropped. — Option A is correct because IPv6 Source Guard, enabled by 'ipv6 verify source', relies on the IPv6 binding table to permit traffic. The binding table is populated by IPv6 Neighbor Discovery (ND) snooping, which learns global unicast and link-local addresses from ND messages. However, OSPFv3 uses its own Hello and LSA packets sent to the multicast address FF02::5, which are not ND messages. Since the router's link-local address is not learned via ND snooping, no binding entry exists, and Source Guard drops the OSPFv3 packets, causing the neighborship to fail.

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. 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 relies on the IPv6 binding table, which is populated by IPv6 Neighbor Discovery (ND) snooping. If the host uses a static IPv6 address and ND snooping is not enabled on the VLAN, the switch never learns the binding, so IPv6 Source Guard drops the traffic as unauthorized. Enabling ND snooping allows the switch to inspect Neighbor Solicitation and Advertisement messages to build the binding table dynamically.

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