Recently was working on DHCP server failover on two VYOS 1.3 routers.
Just to check decided to see how primary and secondary servers communicate with each other.
Have found no traffic on TCP 647 port as stated in documentation while 520 port is alive and working.
In order for the primary and the secondary DHCP server to keep their lease tables in sync, they must be able to reach each other on TCP port 647. If you have firewall rules in effect, adjust them accordingly.
Just curious is it documentation or configuration bug?