Code change:
Actually, the two stub routers might not be connected to the same stub network, and that should be okay. The point is to stabilize the on-link prefix on the AIL if it's provided by a stub router. If stub routers A and B are connected to the same AIL, only one will advertise an on-link prefix. If that one reboots, the other will remember that that prefix is on-link, so it will do the right thing with responses send to hosts on that prefix. The goal is to make sure that the router that rebooted also considers that prefix on-link, even though it's never seen an RA. I think this section needs a bit of work; the clarifications you've requested would be part of that, but it was useful to re-think this based on your comments as well, because I think I see how to make this work in a relatively clean way. If we are not advertising the prefix on the link, because there's a usable prefix on the link, and we get a message from the stub network for an address on the remembered prefix, there are the following possibilities that I can think of: The prefix is a DHCP prefix, meaning it's routable and doesn't belong to the stub router. In this case, the stub router could try to renew it; if that succeeds, it could assume that the prefix is valid on the AIL and forward packets to that prefix to the AIL. If that fails, it assumes the prefix is only reachable through a router, so it forwards the packet using its routing table, and hopes for the best. The prefix is a prefix generated by that stub router. In this case, if the stub router is on a different AIL, it's immaterial, since the prefix isn't routable. So the stub router can assume that that prefix is on-link and forward packets for that prefix to the link; if they reach their intended destination, great. If not, nothing is lost. The prefix is a stub-network-specific prefix. As an example, Thread uses the thread XPANID, which is a 48-bit number unique to a Thread network and its set of credentials, to generate a ULA prefix for the AIL. This means that if there are several stub routers connected to the same AIL and the same Thread network, they will all always advertise the same ULA on the AIL if there's no usable prefix there. This has a really nice benefit of increasing the stability of numbering on the wire in the presence of stub router reboots. This prefix isn't routable, so if the stub router gets connected to a different AIL, there's not much it can do about it. It's not actually clear what to do in this situation, since if there's another stub router still connected to the old AIL, we have ambiguous routing to that prefix on the stub network. In the case of Thread, we don't consider this a serious concern, since thread has similar locality to home networks: if you can't reach the home router, you probably can't reach the thread network you were connected to before either.
Not all aspects described here are fully clear yet, and there's a tension between this text and other PR text that says ULA Site Prefix is used to generate the AIL ULA link prefix. But it's best to merge the new text now and use that as a basis for further review!

Related discussion:
text: A prefix is considered to be advertised on the link if, when a Router Solicit message (RFC4861) is sent, a Router Advertisement message is received in response which contains a prefix information option <del> (RFC4861) </del> <ins> (RFC4861) for that prefix. </ins> After an RA message containing a suitable prefix has been received, it can be assumed for some period of time thereafter that that prefix