hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures an IPv6 ACL to filter…
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?
⚠ Common exam trap
Cisco often tests the distinction between locally generated traffic and transit traffic with respect to outbound ACLs, and the trap here is that candidates assume outbound ACLs filter all traffic leaving the interface, including control plane packets like OSPFv3 hellos.
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 packets are generated by the router's own control plane (the OSPF process) and are not subject to outbound ACL filtering on the interface. Outbound ACLs only filter packets that are being forwarded through the interface, not packets originated by the router itself. Since the ACL is applied outbound, the deny entry for protocol 89 never applies to locally generated OSPFv3 hello packets, so they are sent out, but the inbound side of the neighbor might still be blocked or the adjacency fails due to other reasons—but the core issue here is that outbound ACLs do not filter locally originated traffic.
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.
- ✗
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.
Visual reference
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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