- A
The ACL is applied outbound, but OSPFv3 packets are generated locally and are not subject to outbound ACL filtering.
Correct. Outbound ACLs do not filter locally generated packets, including OSPFv3 Hellos. The ACL must be applied inbound to filter incoming OSPFv3 packets.
- B
The ACL uses 'deny ospf' but OSPFv3 uses protocol 89, which is not matched by 'deny ospf' in IPv6 ACLs.
Why wrong: Incorrect. 'deny ospf' in IPv6 ACLs correctly matches OSPFv3 (protocol 89).
- C
The ACL must include a permit entry for the link-local address of the neighbor, otherwise OSPFv3 packets are dropped.
Why wrong: Incorrect. ACLs can match on source/destination addresses, but OSPFv3 uses link-local addresses; if the ACL permits ipv6 any any, it includes link-local.
- D
The ACL is applied to the interface but the router has 'ipv6 ospf authentication' configured, which changes the packet format and bypasses ACL filtering.
Why wrong: Incorrect. Authentication does not bypass ACL filtering; ACLs are applied before authentication processing.
Quick Answer
The answer is that the ACL is applied outbound, but OSPFv3 packets are generated locally by the router and are not subject to outbound ACL filtering. In Cisco IOS, outbound ACLs only filter traffic that is routed through the interface, not traffic originated by the router itself, such as OSPFv3 hello packets sent to the multicast addresses ff02::5 and ff02::6. This is a common trap on the CCNP ENARSI 300-410 exam, where candidates assume an outbound ACL will block locally generated protocol traffic, but the router’s control plane bypasses outbound interface ACLs entirely. To filter OSPFv3 adjacency formation, the ACL must be applied inbound on the receiving interface, and it must correctly match protocol 89 using the `deny ospf` keyword rather than a numeric protocol number. Memory tip: think “local out, no doubt” — locally originated packets ignore outbound ACLs, so always apply the filter inbound to block OSPFv3.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 an IPv6 ACL to filter OSPFv3 traffic on a router interface. The ACL includes a deny entry for OSPFv3 (protocol 89) followed by a permit ipv6 any any. However, OSPFv3 adjacencies still fail to form over that interface. 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.
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 ACL is applied outbound, but OSPFv3 packets are generated locally and are not subject to outbound ACL filtering.
OSPFv3 uses IPv6 multicast addresses ff02::5 (all OSPF routers) and ff02::6 (all DR/BDR routers). Even if an ACL permits OSPFv3 protocol, the multicast destination may be filtered by other mechanisms or the ACL may not be applied correctly. However, a common edge case is that the ACL is applied inbound and OSPFv3 packets are sourced from link-local addresses, which may not match the ACL source entry if not specified correctly. But the key point: OSPFv3 uses protocol 89 in the IPv6 header, but the ACL must match the protocol number, not the next header field in IPv6. In IOS, the 'ipv6 access-list' uses 'deny ospf' to match OSPFv3, but if the engineer uses 'deny 89' incorrectly, it may not match. The most likely issue: the ACL is applied outbound instead of inbound, or the deny entry incorrectly matches the OSPFv3 packets before the permit.
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.
- ✓
The ACL is applied outbound, but OSPFv3 packets are generated locally and are not subject to outbound ACL filtering.
Why this is correct
Correct. Outbound ACLs do not filter locally generated packets, including OSPFv3 Hellos. The ACL must be applied inbound to filter incoming OSPFv3 packets.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The ACL uses 'deny ospf' but OSPFv3 uses protocol 89, which is not matched by 'deny ospf' in IPv6 ACLs.
Why it's wrong here
Incorrect. 'deny ospf' in IPv6 ACLs correctly matches OSPFv3 (protocol 89).
- ✗
The ACL must include a permit entry for the link-local address of the neighbor, otherwise OSPFv3 packets are dropped.
Why it's wrong here
Incorrect. ACLs can match on source/destination addresses, but OSPFv3 uses link-local addresses; if the ACL permits ipv6 any any, it includes link-local.
- ✗
The ACL is applied to the interface but the router has 'ipv6 ospf authentication' configured, which changes the packet format and bypasses ACL filtering.
Why it's wrong here
Incorrect. Authentication does not bypass ACL filtering; ACLs are applied before authentication processing.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The ACL is applied outbound, but OSPFv3 packets are generated locally and are not subject to outbound ACL filtering. — OSPFv3 uses IPv6 multicast addresses ff02::5 (all OSPF routers) and ff02::6 (all DR/BDR routers). Even if an ACL permits OSPFv3 protocol, the multicast destination may be filtered by other mechanisms or the ACL may not be applied correctly. However, a common edge case is that the ACL is applied inbound and OSPFv3 packets are sourced from link-local addresses, which may not match the ACL source entry if not specified correctly. But the key point: OSPFv3 uses protocol 89 in the IPv6 header, but the ACL must match the protocol number, not the next header field in IPv6. In IOS, the 'ipv6 access-list' uses 'deny ospf' to match OSPFv3, but if the engineer uses 'deny 89' incorrectly, it may not match. The most likely issue: the ACL is applied outbound instead of inbound, or the deny entry incorrectly matches the OSPFv3 packets before the permit.
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.
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Last reviewed: Jun 18, 2026
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