Why

This occasionally occurred in some linux devices which contain lots of neighbor.
My solution is down below:

Solution

Change /etc/sysctl.conf

net.ipv6.neigh.default.gc_thresh1 should be 30 or 50 in default set,

we should extend it and net.ipv6.neigh.default.gc_thresh2, net.ipv6.neigh.default.gc_thresh3 as well.

net.ipv6.route.max_size set ipv6 route size to more then default size in case that route incomplete from kernel route table.

net.ipv6.neigh.default.gc_thresh1 = 512 
net.ipv6.neigh.default.gc_thresh2 = 1024
net.ipv6.neigh.default.gc_thresh3 = 2048
net.ipv6.route.max_size = 2100000000