This read-only Counter32 tallies the number of dropped DHCPv6 messages found illegal under Neighbor Binding Integrity checks on the interface since the counter was last reset. An admin cares because DHCPv6 packets failing the same class of sanity checks as the ICMPv6 case, applied to DHCPv6 traffic instead, so watching this counter helps spot attacks, misconfigured neighboring devices, or an overly strict policy. For example, if this counter on port Gi1/0/3 climbs rapidly right after a new device joins the LAN, it is a strong clue that device is the source of the offending IPv6 traffic on that link.