IPv6 neighbor flaps for ISP router are occurring frequently

From: Chris Ross <cross+freebsd_at_distal.com>
Date: Mon, 16 Sep 2024 20:05:39 UTC
Hello.  Following the earlier thread "DHCPv6 IA_PD - how-to” I have been
bringing up a new gateway router for my network.  With Roy’s help, I
have IPv6 working as expected, but I am seeing something that I suspect
is unusual.  Roy doesn’t think it’s specific to dhcpcd, as it is caused
by routing changes that dhcpcd is merely reacting to.

While dhcpcd is running, it repeatedly logs the following to daemon.log:

Sep 16 15:55:53 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is unreachable
Sep 16 15:55:54 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is reachable again
Sep 16 15:55:54 logrus dhcpcd[50070]: vlan0: soliciting an IPv6 router
Sep 16 15:56:25 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is unreachable
Sep 16 15:56:25 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is reachable again
Sep 16 15:56:25 logrus dhcpcd[50070]: vlan0: soliciting an IPv6 router
Sep 16 15:56:57 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is unreachable
Sep 16 15:56:58 logrus dhcpcd[50070]: vlan0: soliciting an IPv6 router
Sep 16 15:56:58 logrus dhcpcd[50070]: vlan0: Router Advertisement from fe80::3e8a:b0ff:fe3e:4dce
Sep 16 15:57:32 logrus dhcpcd[50070]: vlan0: fe80::3e8a:b0ff:fe3e:4dce is unreachable
Sep 16 15:57:32 logrus dhcpcd[50070]: vlan0: soliciting an IPv6 router
Sep 16 15:57:32 logrus dhcpcd[50070]: vlan0: Router Advertisement from fe80::3e8a:b0ff:fe3e:4dce

During this same time frame, “route monitor” shows:

15:55:53.573 PID    0 delete neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state PROBE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:55:54.002 PID    0 add/repl neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state REACHABLE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:56:25.148 PID    0 delete neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state PROBE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:56:25.212 PID    0 add/repl neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state REACHABLE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:56:57.406 PID    0 delete neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state PROBE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:56:58.967 PID    0 add/repl neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state REACHABLE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:57:32.325 PID    0 delete neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state PROBE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  15:57:34.143 PID    0 add/repl neigh fe80::3e8a:b0ff:fe3e:4dce%vlan0 state REACHABLE lladdr 3c:8a:b0:3e:4d:ce iface vlan0  

My concern is that this is sending RS’s to my provider far too often.
Can anyone advise if this is normal, and assuming not, what might
be wrong?

Trimmed dhcpcd.conf is:

duid
persistent
vendorclassid
option classless_static_routes
option rapid_commit
require dhcp_server_identifier
slaac private
noipv6rs
noipv4
noipv4ll
allowinterfaces vlan0
interface vlan0
  ipv6only
  ipv6rs
  ipv6ra_autoconf
  ia_pd 0/::/56 intnet1/42 intnet2/56

Thanks.

                        - Chris