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

Karl Auer kauer at
Tue Nov 2 06:55:21 CDT 2010

On Tue, 2010-11-02 at 10:51 +0000, Tim Franklin wrote:
> > That breaks the IPv6 spec. Preferred and valid lifetimes are there
> > for a reason.
> And end-users want things to Just Work.  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.

But - preferred and valid lifetimes do *exactly that*. The address is
fully usable up to the end of the preferred lifetime. It is then
deprecated (but not unusable) until the end of the valid lifetime. Only
after the valid lifetime does it become unusable. DHCPv6 lifetimes are
exactly the same as RA lifetimes - and of course there is nothing that
says the RA lifetimes have to be the same as the DHCPv6 lifetimes
(though some sensible relationship would be advisable).

So loss of connectivity to the upstream is not going to blow away a home
network. It will keep working fine, even if the upstream goes away for a
while. It's up to the upstream to use lifetimes that are a good
compromise between flexibility and stability.

About the only hack I can see that *might* make sense would be that home
CPE does NOT honour the upstream lifetimes if upstream connectivity is
lost, but instead keeps the prefix alive on very short lifetimes until
upstream connectivity returns.

Regards, K.

Karl Auer (kauer at                   +61-2-64957160 (h)                   +61-428-957160 (mob)

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