An emergency-management office applies an IPv6 traffic filter on camera subnets after a dual-stack cutover, but some ICMPv6 neighbor-discovery traffic breaks unexpectedly. What troubleshooting awareness is most appropriate?

When an emergency-management office applies an IPv6 traffic filter on camera subnets after dual-stack cutover and ICMPv6 neighbor-discovery traffic breaks, the appropriate troubleshooting awareness is that IPv6 filters must account for required ICMPv6/ND-related traffic or use carefully ordered permits; overly broad denies can break neighbor discovery and related functions. Unlike IPv4 ARP, IPv6 ND uses ICMPv6 messaging that must be allowed for local connectivity. The correct awareness works because a filter that "looks right" for HTTPS/camera payloads can still black-hole on-link resolution if ND is denied. Claiming IPv6 ACLs never match ICMPv6 so ND failure must be OSPFv3 area-type only fails because IPv6 ACLs can and do match ICMPv6, and ND breakage is a classic filter symptom independent of OSPFv3 area design. Asserting IPv6 traffic filters automatically permit all link-local protocols regardless of deny entries fails conceptually: explicit or implicit denies can drop needed ND/ICMPv6 unless permitted. Saying camera subnets cannot use IPv6 ACLs until MPLS TE tunnels exist end to end fails because IPv6 interface ACLs do not require MPLS TE as a prerequisite. Exam caveat: permitting "ICMP" generically may still be too coarse or too narrow depending on types/codes needed; know which ND messages the civic cameras and routers require on-link. Operational check: inspect the IPv6 ACL for denies affecting ICMPv6, temporarily permit necessary ND-related traffic, clear neighbor state, retest camera reachability, and confirm ND entries form while keeping the intended security posture for nonessential ICMPv6.