hardMultiple Choice
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
Denying ICMPv6 types 133–137 blocks Router Solicitation, Router Advertisement, Neighbor Solicitation, Neighbor Advertisement and Redirect, which Neighbor Discovery depends on for address resolution and router discovery. The ACL's implicit deny-all therefore severs ND, so the interface cannot resolve neighbours or maintain IPv6 connectivity.
- ✗
The ACL must be applied outbound, not inbound, to allow ND packets to be sent.
Why it's wrong here
ND relies on inbound Neighbor Solicitation and Neighbor Advertisement messages, so direction is not the fault; applying the ACL outbound would still block the inbound NS/NA the router must receive. Outbound filtering is chosen when the requirement is to restrict traffic leaving a segment, not to protect inbound protocol processing.
- ✗
The router needs to have 'ipv6 nd suppress' configured to bypass ACL filtering for ND packets.
Why it's wrong here
'ipv6 nd suppress' stops the router sending Router Advertisements on an interface; it does not exempt ND from ACL evaluation, so NS and NA remain filtered. Suppression is used on interfaces where hosts must not autoconfigure addresses from the router, such as towards upstream providers.
- ✗
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
A blanket permit ipv6 any any would defeat the stated requirement to deny all other IPv6 traffic, and ND is not restored by ordering alone because the ACL already denies the specific ICMPv6 ND types. Such a permit is used when the intent is simply to allow all IPv6 with exceptions.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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