IPv6) I can scan your v6 /64 subnet, and your router will have to send out NDP NS for every host I scan. If it requires "incomplete" entries in its table, I will use them all up, and NDP learning will be broken. Typically, this breaks not just on that interface, but on the entire router. This is much worse than the v4/ARP sitation.
I'm guessing you're referring to this paragraph of RFC 4861: " When a node has a unicast packet to send to a neighbor, but does not know the neighbor's link-layer address, it performs address resolution. For multicast-capable interfaces, this entails creating a Neighbor Cache entry in the INCOMPLETE state and transmitting a Neighbor Solicitation message targeted at the neighbor. The solicitation is sent to the solicited-node multicast address corresponding to the target address. " <http://tools.ietf.org/html/rfc4861#section-7.2.2> It's worth noting that nothing in this paragraph is normative (there's no RFC 2119 language), so implementations are free to ignore it. I haven't read the NIST document, but it wouldn't conflict with the RFC if they recommended ignoring this paragraph and just relying on the ND cache they already have when a packet arrives. --Richard