IPv6 default route flapping

[Jan Schaumann <jschauma%netmeister.org@localhost>]

Hi,

I'm experiencing a problem with my NetBSD AWS EC2
instance flapping the default IPv6 route:

The system is an amd64 EC2 instance using:

# uname -a
NetBSD netbsd 9.99.78 NetBSD 9.99.78 (GENERIC) #0: Fri Jan 22 00:44:55 UTC 2021
mkrepro%mkrepro.NetBSD.org@localhost:/usr/src/sys/arch/amd64/compile/GENERIC amd64

I have 'dhcpcd=YES' and the system comes up with an
IPv6 lease:

# ifconfig xennet0
xennet0: flags=0x8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        capabilities=0x17c00<TCP4CSUM_Rx,TCP4CSUM_Tx,UDP4CSUM_Rx,UDP4CSUM_Tx>
        capabilities=0x17c00<TCP6CSUM_Rx,UDP6CSUM_Rx>
        enabled=0
        ec_capabilities=0x5<VLAN_MTU,JUMBO_MTU>
        ec_enabled=0
        address: 0e:61:d2:b8:e6:b1
        inet 10.10.0.47/26 broadcast 10.10.0.63 flags 0
        inet6 fe80::7e12:b688:167c:785f%xennet0/64 flags 0 scopeid 0x1
        inet6 2600:1f18:400c:b800:bc3c:63cc:7e5d:1f96/128 flags 0
# route get -inet6 default
   route to: ::
destination: ::
       mask: default
    gateway: fe80::caa:49ff:feaf:1815%xennet0
 local addr: fe80::7e12:b688:167c:785f%xennet0
  interface: xennet0
      flags: 0x43<UP,GATEWAY,DONE>
 recvpipe  sendpipe  ssthresh  rtt,msec    rttvar hopcount      mtu     expire
       0         0         0         0         0      0         0         0 


Everything's working fine, but just about every 10
seconds or so, the default IPv6 route drops, it
solicits a default route, and then immediately gets
the same RA:

Apr 20 01:32:32 netbsd dhcpcd[17397]: xennet0: soliciting an IPv6 router
Apr 20 01:32:34 netbsd dhcpcd[17397]: xennet0: Router Advertisement from fe80::caa:49ff:feaf:1815
Apr 20 01:32:35 netbsd dhcpcd[17397]: xennet0: fe80::caa:49ff:feaf:1815 is unreachable
Apr 20 01:32:35 netbsd dhcpcd[17397]: xennet0: soliciting an IPv6 router
Apr 20 01:32:44 netbsd dhcpcd[17397]: xennet0: fe80::caa:49ff:feaf:1815 is reachable again
Apr 20 01:32:52 netbsd dhcpcd[17397]: xennet0: fe80::caa:49ff:feaf:1815 is unreachable
Apr 20 01:32:52 netbsd dhcpcd[17397]: xennet0: soliciting an IPv6 router
Apr 20 01:32:54 netbsd dhcpcd[17397]: xennet0: Router Advertisement from fe80::caa:49ff:feaf:1815
Apr 20 01:32:55 netbsd dhcpcd[17397]: xennet0: fe80::caa:49ff:feaf:1815 is unreachable
Apr 20 01:32:56 netbsd dhcpcd[17397]: xennet0: soliciting an IPv6 router


Note: other OS instances on the same VPC (i.e.,
getting the same network information and using the
same default route) do not see this problem -- it only
appears on NetBSD.

Does anybody know what might be causing this?

-Jan