Failover IPv6 with multiple PA prefixes (Was: IPv6 fc00::/7 - Unique local addresses)

Mark Smith nanog at
Tue Nov 2 07:53:27 CDT 2010

On Tue, 2 Nov 2010 10:51:44 +0000 (GMT)
Tim Franklin <tim at> wrote:

> >> Your home gateway that talks to your internet connection can either
> >> get it via DHCP-PD or static configuration. Either way, it could
> >> (should?) be set up to hold the prefix until it gets told something
> >> different, possibly even past the advertised valid time.
> > 
> > That breaks the IPv6 spec. Preferred and valid lifetimes are there
> > for a reason.
> And end-users want things to Just Work. 

And I want their networks to work so well that I don't even want them
to rely in an ISPs addressing being available, valid, or even having an
ISP - which could easily be the case if they go and sign up for a new
broadband service, bring home brand new CPE, yet don't get ISP service
connectivity for 5 to 10 business days. Surely they should be able to
hook up their internal network and have their TV talking to their
computer or NAS during this period without an Internet service. The ISP
in question may not be prepared to give them a permanent GUA address at
the time of sign up, because the ISP may wish to have static addressing
as a product distinguisher for SOHO/SME products, or have the
flexibility of phasing semi-dynamic addressing in and out over time to
suit their IPv6 address management requirements.

> The CPE vendor that finds a hack that lets the LAN carry on working
 while the WAN goes away and manages to slap the "With Home Network
 Resilience!" label on the box correctly will presumably do quite
 nicely out of it.
> For this kind of site, I can't see what is *actually* going to break if the CPE keeps sending RAs for the prefix beyond the valid lifetime while the WAN is down.  As long as it advertises a short valid lifetime itself, such that if the real prefix changes[0] when the WAN comes back up it can renumber everything on the LAN quickly, it looks a lot like a "Just Works" scenario to me...

Prefix lifetimes don't work that way - there is no such thing as a
"flash" renumbering. The goal was to be able to phase new
addressing in, transition to it as either older communcations
sessions cease (e.g. TCP connections), or new ones are established, then
phase out the old addressing over a more significant time period than
one measured in minutes or seconds.


> Regards,
> Tim.
> [0] Which it won't, of course, because residential users are going to get proper static connections by default, rather than another round of "business class" price-gouging :)

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