hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures an IPv6 ACL on a router…
An engineer configures an IPv6 ACL on a router interface to permit only specific ICMPv6 types (e.g., echo request and echo reply) and deny all other IPv6 traffic. After applying the ACL inbound, the router stops forming IPv6 neighbor discoveries (ND) and the interface loses IPv6 connectivity. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the candidate's awareness that IPv6 Neighbor Discovery uses specific ICMPv6 types (133-137) which must be explicitly permitted in ACLs, as they are not automatically allowed like in IPv4 ARP.
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 denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors.
The ACL denies ICMPv6 types 133-137, which are essential for Neighbor Discovery (ND) processes such as Router Solicitation (133), Router Advertisement (134), Neighbor Solicitation (135), Neighbor Advertisement (136), and Redirect (137). Without permitting these types, the router cannot resolve IPv6 neighbors or maintain IPv6 connectivity, as ND is fundamental to IPv6 operation.
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 denies ICMPv6 types 133-137, which are required for Neighbor Discovery, causing the router to fail to resolve neighbors.
Why this is correct
Correct. ND uses ICMPv6 types 133-137; denying them breaks IPv6 connectivity.
- ✗
The ACL must be applied outbound, not inbound, to allow ND packets to be sent.
Why it's wrong here
Incorrect. ND packets are both sent and received; inbound ACL filters incoming packets, which include ND solicitations and advertisements.
- ✗
The router needs to have 'ipv6 nd suppress' configured to bypass ACL filtering for ND packets.
Why it's wrong here
Incorrect. There is no such command; ND packets are subject to ACL filtering unless explicitly permitted.
- ✗
The ACL should use 'permit ipv6 any any' before the deny statements to allow ND, but the engineer placed it after.
Why it's wrong here
Incorrect. Even if 'permit ipv6 any any' is placed, it would permit all traffic, including ND, but the question states the ACL permits only ICMPv6 echo types, so ND is still denied.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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