[toast0]

> What I've been told is both ISP's v6 prefix should be advertised with different priorities to the clients

That basically doesn't work with real clients. They'll do dumb stuff like use address from provider A to send through the router advertising addresses from provider B. And take forever to do anything in response to prefixes that are advertised as no longer usable or simply no longer advertised.

[codatory]

Is this conjecture or something you’ve tested or known to be tested? V6 devices are actually expected to be able to understand multiple route advertisements and I know for sure they do properly understand the mix of link-local, ULA and public prefixes.

[toast0]

I tested it; I was trying to get failover (preferably automated failover) between DSL and LTE on IPv6. Should be simple: advertise from DSL as normal priority (would do high priority, but I can't change how the modem advertises it), advertise from the LTE as low priority, somehow make the DSL modem advertise deprecated or at least stop advertising when it's disconnected.

V6 devices are expected to understand that and do the right thing, but Windows (10) doesn't, Linux was worse, and I don't remember what Android did and I didn't get around to testing FreeBSD, and that's all the OSes I have.

If you've got experience otherwise, I'd love to know, one of these days I need to setup IPv6 again, but what I'd really like to do is too much work, so I'm IPv4 only for the foreseeable future.