Comment 8 for bug 1800836

fw01/02 have bond0.21 that is setup to have fe80::1 as the VIP used as the network gateway:

root@fw01:~# ip -6 a show bond0.21
8: bond0.21@bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2620:a:b:21::2/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::210:18ff:fe77:b558/64 scope link
       valid_lft forever preferred_lft forever

# fw02 is currently primary/master
root@fw02:~# ip -6 a show bond0.21
8: bond0.21@bond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2620:a:b:21::3/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::1/64 scope link nodad
       valid_lft forever preferred_lft forever
    inet6 fe80::210:18ff:febe:6750/64 scope link
       valid_lft forever preferred_lft forever

fw01/02 /etc/radvd.conf looks like this:

interface bond0.21
{
  AdvSendAdvert on;
  MaxRtrAdvInterval 30;

  prefix 2620:a:b:21::/64
  {
  };
};

and radvd only runs on the primary/master fw (02 ATM).

After a failover, Bionic clients using netplan/systemd-networkd will have a bogus default nexthop like this:

$ ip -6 ro
2620:a:b:21::/64 dev eth0 proto ra metric 1024 pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
default proto ra metric 1024
 nexthop via fe80::1 dev eth0 weight 1
 nexthop via fe80::210:18ff:fe77:b558 dev eth0 weight 1
 nexthop via fe80::210:18ff:febe:6750 dev eth0 weight 1

Preventing them from communicating properly. To fix this, one has to manually do this:

sudo ip -6 ro del default proto ra && sudo netplan apply

Which then give the expected route entries like:

$ ip -6 ro
2620:a:b:21::/64 dev eth0 proto ra metric 1024 pref medium
fe80::/64 dev eth0 proto kernel metric 256 pref medium
default via fe80::1 dev eth0 proto ra metric 1024 pref medium

On the other hand, machines using ifupdown (Xenial or Bionic) in the same network segment have no problem keeping only fe80::1 as the default nexthop.