>>>>> Imagine a setup with *two* routers.  One of them has broken Internet,
>>>>> the other is working.  How can the hosts decide if both keep announcing
>>>>> themselves as "I can reach anything"?
>>>> in the general case the host still has to take the 'I can reach anything' 
>>>> announcement with a pinch of salt.
>>>> and it should be able to try both (or more) connections and react 
>>>> accordingly when one fails.
>>> ...which is the default host behaviour if the OS supports RFC4861.
>>> Sadly some "user friendly" network mangers breaks this and setting a
>>> static route with a better metric to just one(!) router.
>> not really. that only covers the first hop. any failure anywhere else along 
>> the path would not be dealt with by 4861.
> Yes, of course, you're right.
> But coming back to Brians problem.
> The behavior of his router is confirm to RFC7084:
> "G-4:  By default, an IPv6 CE router that has no default router(s) on
>         its WAN interface MUST NOT advertise itself as an IPv6 default
>         router on its LAN interfaces.  That is, the "Router Lifetime"
>         field is set to zero in all Router Advertisement messages it
>         originates [RFC4861]."
> But this would break local connectivity if the router does not also
> fulfill L-3
> "L-3:   An IPv6 CE router MUST advertise itself as a router for the
>          delegated prefix(es) (and ULA prefix if configured to provide
>          ULA addressing) using the "Route Information Option" specified
>          in Section 2.3 of [RFC4191].  This advertisement is
>          independent of having or not having IPv6 connectivity on the
>          WAN interface."
> So far so good. But if the host doesn't honour the RIO then again local
> connectivity is broken.
> So take a look at the reasons for this. The second paragraph of 3.2.1 says:
> "At the time of this writing, several host implementations do not
>   handle the case where they have an IPv6 address configured and no
>   IPv6 connectivity, either because the address itself has a limited
>   topological reachability (e.g., ULA) or because the IPv6 CE router is
>   not connected to the IPv6 network on its WAN interface.  To support
>   host implementations that do not handle multihoming in a multi-prefix
>   environment [MULTIHOMING-WITHOUT-NAT], the IPv6 CE router should not,
>   as detailed in the requirements below, advertise itself as a default
>   router on the LAN interface(s) when it does not have IPv6
>   connectivity on the WAN interface or when it is not provisioned with
>   IPv6 addresses.  For local IPv6 communication, the mechanisms
>   specified in [RFC4191] are used."
> At the end, the whole behavior is because some host have problems in
> handling situations where they have an IPv6 address configured and now
> internet connectivity. But the solution to this requires that the host
> is able to understand and work with RIO options, which seams to be "at
> the time" not generally the case.
> Do we replace one evil by another?

Yes, we did.
This work was done at the time of "host IPv6 brokenness".
Perhaps time to revisit those decisions?

Best regards,

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