I have to use statefull dhcpv6 client to receive prefix and assign it to downstream router. But when upstream prefix changes,it can not respond immediately, and network goes down for a while.
Could you describe the network topology a bit better?
The OpenWrt router (vendor, model OpenWrt version?) is the main internet router and delegates a prefix to a downstream router, or gets a prefix delegated from the upstream router of the ISP?
What is the output of
uci export network; uci export dhcp ?
Router A connects to ISP by PPPoE, and gets a /56 prefix on WAN interface, I set
ip6assign '60' on its LAN interface, then connect Router B's WAN to A's LAN, set
ip6assign '64' on B's LAN interface.
Now the problem is, hosts connected to B can not get update immediately when A reconnects and gets a new prefix.
What is the delay more or less?
You could try to make the RA timings stricter as described here to reduce the window with invalid addresses.