Consider the following: A node tries to forward a packet to another
host for which it does not know the route yet. It assumes it to be
on-link and starts a neighbor solicitation, putting the node address
in the destinatio cache.
Later the route is known (via a second hop) but the host is still in
the NIB.
The result is that gnrc_ipv6_nib_get_next_hop_l2addr() ends up in the
"nib: %s is in NC or on-link, start address resolution" case and does
not attempt to resolve the route.
This results in the host remaining unreachable even though now a route
is present.
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