radvd reports an error about an invalid all-zeros prefix in syslog, but continues to start up and work fine for that prefix, so there is no problem with it, just a erroneous log.
This is reported in debian since 2018, still open: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=891324
and fixed upstream in radvd 2.18 https://github.com/reubenhwk/radvd/commit/b37baa1137d0bd5b9cceb2e447550f1c0a105ac6
router-advert { interface eth2 { prefix ::/64 { } } }
Jun 25 12:35:00 vyos systemd[1]: Starting Router advertisement daemon for IPv6... Jun 25 12:35:00 vyos radvd[3066]: invalid all-zeros prefix in /run/radvd/radvd.conf, line 14 Jun 25 12:35:00 vyos radvd[3066]: config file, /run/radvd/radvd.conf, syntax ok Jun 25 12:35:00 vyos radvd[3067]: version 2.17 started Jun 25 12:35:00 vyos radvd[3067]: invalid all-zeros prefix in /run/radvd/radvd.conf, line 14 Jun 25 12:35:00 vyos systemd[1]: Started Router advertisement daemon for IPv6.