Port 546 is reserved for the client-side of the Neighbor Discovery Protocol (NDP).
This is used for communication between IPv6 nodes (such as a device and its router)
to discover and configure network information (such as IP address).
Router Advertisement (RA) messages sent by routers use port 547 (router-side), and
devices use port 546 to receive these messages.
See https://forum.cloudron.io/topic/13566/infomaniak-ipv6-issues/61
7f87af5a08