Over-utilisation of v6 neighbour slots

[Erik Kline]

> Maybe we need a "neigbour un-advert" ICMPv6 message that the old addresses
> could be torn down with.

The routers could issue MLD queries either to addresses individually
[1] or to the link in general [2] and prune addresses on which nobody
is listening (following something like 3 tries to verify...).

[1]  pick MACs with >= THRESHOLD IPv6 addresses associated, or timer-based

[2]  should probably be a function of the total number of unique L2
neighbors as MLD Reports from everyone could be a lot of traffic