Reproducing steps:
set service dhcpv6-server shared-network-name VyOS-DHCPv6 subnet 2001:db8:3456::/64 address-range start 2001:db8:3456::100 stop '2001:db8:3456::1ff' set service dhcpv6-server shared-network-name VyOS-DHCPv6 subnet 2001:db8:3456::/64 name-server '2001:db8:daad::1' set service dhcpv6-server shared-network-name VyOS-DHCPv6 subnet 2001:db8:3456::/64 prefix-delegation start 2001:db8:290::500 prefix-length '64' set service dhcpv6-server shared-network-name VyOS-DHCPv6 subnet 2001:db8:3456::/64 prefix-delegation start 2001:db8:290::500 stop '2001:db8:290::5ff'
It allows to apply the setting, but DHCPv6 server does not start and die with error in logs
Apr 23 11:30:01 vyos systemd[1]: Starting ISC DHCP IPv6 server... Apr 23 11:30:01 vyos dhcpd[8404]: /run/dhcp-server/dhcpdv6.conf line 13: network mask too short Apr 23 11:30:01 vyos dhcpd[8404]: prefix6 2001:db8:290::500 2001:db8:290::5ff/64; Apr 23 11:30:01 vyos dhcpd[8404]: ^ Apr 23 11:30:01 vyos dhcpd[8404]: Configuration file errors encountered -- exiting Apr 23 11:30:01 vyos dhcpd[8404]: Apr 23 11:30:01 vyos dhcpd[8404]: If you think you have received this message due to a bug rather Apr 23 11:30:01 vyos dhcpd[8404]: than a configuration issue please read the section on submitting Apr 23 11:30:01 vyos dhcpd[8404]: bugs on either our web page at www.isc.org or in the README file Apr 23 11:30:01 vyos dhcpd[8404]: before submitting a bug. These pages explain the proper Apr 23 11:30:01 vyos dhcpd[8404]: process and the information we find helpful for debugging. Apr 23 11:30:01 vyos dhcpd[8404]: Apr 23 11:30:01 vyos dhcpd[8404]: exiting.