Reg:IPv6 Default Router List

In IPv6 environment,when we have many routers sending out router
advertisements,one of these routers will be configured as the default
router.
When this default router is removed from the network,how do we
determine that it is no longer reachable?
Does "neighbour unreachability detection" handle this ,detect it
within 38 seconds and delete that router entry automatically from the
default router list?
I notice that the entry remains in the default router list as long as
the router "lifetime " value doesnt expire. I would like to know
what
is the idea behind having that entry if the router is not in the
network?


Thanks