Why does an IPv6 neighbour router status become STALE? How can I avoid it?

I have a VM on a host with bridged networking (hence, with its own MAC address). Both host and VM run CentOS. Their network is managed by simple /etc/sysconfig/network-scripts/ifcfg-enpXsY files. IPv4 works just fine. I have assigned an IPv6 address to the VM (the host also has one) which is routed correctly in the data centre. Most connections use IPv4, however (no DNS AAAA entry for the machine yet, still testing IPv6). When I boot up the VM it has full IPv6 connectivity. However, **after a while IPv6 connectivity stops working** (IPv6 magic?). I have narrowed to problem down to neighbour (ARP/NDISC cache) data: Not working, cannot ping or connect by IPv6 in or out: # ip -6 neighbour fe80::1 dev enp1s2 lladdr 0c:86:72:2e:04:28 router STALE Fix/workaround to refresh the cache: # ip -6 neighbour flush dev enp1s2 # ip -6 neighbour (empty, as expected) Then ping6 the host from within the VM to fill the cache: # ping6 2912:1375:23:9a6c::2 PING 2912:1375:23:9a6c::2(2912:1375:23:9a6c::2) 56 data bytes 64 bytes from 2912:1375:23:9a6c::2: icmp_seq=1 ttl=64 time=2.35 ms 64 bytes from 2912:1375:23:9a6c::2: icmp_seq=2 ttl=64 time=0.468 ms ^C # ip -6 neighbour fe80::1 dev enp1s2 lladdr 0c:86:72:2e:04:28 router REACHABLE 2912:1375:23:9a6c::2 dev enp1s2 lladdr 08:21:4b:b7:f8:31 DELAY IPv6 neighbour/ARP table restored to validity and connectivity is working in and out! **So my questions are:** 1. Why does the cache become stale? 2. What can I do to avoid it? Of course I could run those commands in a cron job (how often?) but I suppose that cannot really be needed for IPv6 to work in general? PS: I used a script for tests: **The IPv6 stack breaks down about every 20 minutes**. Can that be explained by RFCs?