IPv6 Neighbor Discovery includes Neighbor Unreachability Detection.
That function is very useful when a host has an alternative neighbor
-- for instance, when there are multiple default routers -- since it
allows the host to switch to the alternative neighbor in a short
time. By default, this time is 3 seconds after the node starts
probing. However, if there are no alternative neighbors, this
timeout behavior is far too impatient. This document specifies
relaxed rules for Neighbor Discovery retransmissions that allow an
implementation to choose different timeout behavior based on whether
or not there are alternative neighbors. This document updates RFC
4861.