Re: [v6ops] ULA precedence [Thoughts about wider operational input]

[Erik Auerswald <[e-mail]>]

Hi Brian,

On Mon, Apr 25, 2022 at 08:17:45PM +1200, Brian Carpenter wrote:
> On Mon, 25 Apr 2022, 19:49 Erik Auerswald, <[e-mail]>
> wrote:
> > ....
> >
> > Using different first hop gateways for GUA and ULA is another
> > problem.  It would be nice if hosts would (optionally) not consider
> > a gateway announcing a ULA prefix as default gateway, but only for
> > ULA reachability.
> 
> I think RFC8028 solves that.

Thanks!

The solution from RFC 8028 is fine when using ULA for local connectivity
and GUA for global connectivity, even with different first hop gateways
for GUA and ULA.

I think RFC 8028 section 3.2 paragraph 2 means that if there is only one
first hop gateway announcing ULA prefixes, GUA reachability would still
be assumed, and the first hop gateway used as default gateway.

It does not prevent the host from attempting to reach a GUA destination
using a ULA source address if the only first hop gateway only announces
ULA prefixes.  This is also fine, because RFC 6724 specifies to prefer
IPv4 over ULA, allowing fall-back to IPv4 for global connectivity, if
available.  (All this does not matter without any global connectivity).

To be explicit: If we could have configured the first hop routers in a
way that both announced a ULA prefix on-link and prevented the hosts
from using the router as default gateway, but still for ULA
connectivity, that would have been a possible workaround for lack of RFC
6724 support in the hosts back then.  The RFC 6724 solution would have
worked, too.

Kind regards,
Erik
-- 
Dipl.-Inform. Erik Auerswald
Gesellschaft für Fundamental Generic Networking mbH
Geschäftsführung: Volker Bauer, Jörg Mayer
Gerichtsstand: Amtsgericht Kaiserslautern - HRB: 3630