- A
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
RA Guard host role restricts traffic to ND and DHCP only, blocking OSPFv3.
- B
OSPFv3 uses multicast address FF02::5 which is filtered by RA Guard by default.
Why wrong: RA Guard does not filter OSPFv3 multicast unless configured to do so.
- C
RA Guard changes the MAC address of the router, causing OSPFv3 neighbor to be unreachable.
Why wrong: RA Guard does not modify MAC addresses.
- D
The router must send Router Advertisements for OSPFv3 to work, and RA Guard blocks them.
Why wrong: OSPFv3 does not require RAs.
Question 390 of 2,152
RA Guard Blocking OSPFv3 — Adjacency Failure | Cisco CCNP ENARSI 300-410
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 RA Guard on a switch port connected to a router running OSPFv3. Unexpectedly, OSPFv3 neighbor adjacencies fail to form on that link. Which is the most likely explanation?
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.
Quick Answer
The answer is that RA Guard configured with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, which inadvertently blocks OSPFv3 hello packets sent to the multicast address FF02::5. This occurs because the RA Guard policy, when set to host mode, treats the connected router as an end device and filters out non-essential IPv6 traffic, including the OSPFv3 hellos required for adjacency formation. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how security features like RA Guard can interfere with routing protocols—a common trap is assuming RA Guard only blocks Router Advertisements, when in fact the device-role host parameter imposes a broader filter. Remember the memory tip: "Host mode halts hellos"—if a router port is misclassified as a host, OSPFv3 adjacencies will fail because the switch drops the protocol traffic it deems unnecessary for an end device.
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
✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
RA Guard with the device-role host command is designed to block all IPv6 traffic except Neighbor Discovery (ND) and DHCPv6 on the port. This includes OSPFv3 hellos, which use multicast address FF02::5. Since OSPFv3 relies on these hellos to form neighbor adjacencies, blocking them prevents adjacency establishment.
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.
- ✓
RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos.
Why this is correct
RA Guard host role restricts traffic to ND and DHCP only, blocking OSPFv3.
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 address FF02::5 which is filtered by RA Guard by default.
Why it's wrong here
RA Guard does not filter OSPFv3 multicast unless configured to do so.
- ✗
RA Guard changes the MAC address of the router, causing OSPFv3 neighbor to be unreachable.
Why it's wrong here
RA Guard does not modify MAC addresses.
- ✗
The router must send Router Advertisements for OSPFv3 to work, and RA Guard blocks them.
Why it's wrong here
OSPFv3 does not require RAs.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that RA Guard only filters Router Advertisements, when in fact the device-role host option blocks all non-ND/DHCPv6 traffic, including OSPFv3 hellos.
Detailed technical explanation
How to think about this question
RA Guard (RFC 6105) is a first-hop security feature that prevents rogue Router Advertisements by enforcing a device role on a switch port. When configured with device-role host, the switch drops any IPv6 traffic that is not Neighbor Discovery (ND) or DHCPv6, including routing protocol packets like OSPFv3 hellos (sent to FF02::5). In real-world deployments, this can inadvertently break OSPFv3 if the port is misconfigured as host instead of router, requiring careful role assignment per interface.
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: RA Guard with device-role host drops all IPv6 traffic except Neighbor Discovery and DHCP, including OSPFv3 hellos. — RA Guard with the device-role host command is designed to block all IPv6 traffic except Neighbor Discovery (ND) and DHCPv6 on the port. This includes OSPFv3 hellos, which use multicast address FF02::5. Since OSPFv3 relies on these hellos to form neighbor adjacencies, blocking them prevents adjacency establishment.
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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Last reviewed: Jul 4, 2026
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