you should advertise a local prefix (anything in fd00::/8) in your network and it should just work. no need to use the isp-provided prefix for lan.

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justaboutanyone a day ago | parent | next [-]

There are some address source selection problems if you're still using any ipv4 for the local services https://blog.ipspace.net/2022/05/ipv6-ula-made-useless/

	▲	jeroenhd a day ago \| parent [-]
		Are those problems? If either addressing method works and is reachable, who cares which one end up getting used first?

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silb a day ago | parent | prev [-]

For IPv6, multiple addresses on an interface is the norm: an interface has both a public address from your ISP (replacing IPv4 NAT) and a unique local address (replacing stable IPv4 RFC 1918 LAN addresses).