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

Mark Andrews marka at
Sun Oct 31 21:43:39 CDT 2010

In message <AANLkTimsB6Uj-jpogLg08Q-RZDUB-+C9c5KMzcKTQKmQ at>, Chri
stopher Morrow writes:
> On Sun, Oct 31, 2010 at 2:01 PM, George Bonser <gbonser at> wrote:
> >> ula really never should an option... except for a short lived lab,
> >> nothing permanent.
> >
> > I have a few candidate networks for it. =A0Mostly networks used for
> > clustering or database access where they are just a flat LAN with no
> > "gateway". =A0No layer 3 gets routed off that subnet and the only things
> > talking on it are directly attached to it.
> why not just use link-local then?

If you had actually every tried to use link-local then you would know why
you don't use link-local.

> eventually you'll have to connect
> that network with another one, chances of overlap (if the systems
> support real revenue) are likely too high to want to pay the
> renumbering costs, so even link-local isn't a 100% win :(
> globally-unique is really the best option all around.

2^40 is 1099511627776.  The chances of collision are so low that
one really shouldn't worry about it.  You are millions of times
more likely of dieing from a asteroid 1-in-500,000[1].

If you merge thousands of ULA and don't consolidate then you start
to have a reasonable chance of collision.  Even if you do have
colliding ULA prefixes you don't necessarially have colliding subnets
when merging companies.  Just allocate subnet randomly.  It's not
like 2^16 internal subnets is going to be a major routing problem.


Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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