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

Owen DeLong owen at delong.com
Mon Nov 1 04:25:33 UTC 2010


On Oct 31, 2010, at 7:43 PM, Mark Andrews wrote:

> 
> In message <AANLkTimsB6Uj-jpogLg08Q-RZDUB-+C9c5KMzcKTQKmQ at mail.gmail.com>, Chri
> stopher Morrow writes:
>> On Sun, Oct 31, 2010 at 2:01 PM, George Bonser <gbonser at seven.com> 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.
> 
I use link local often for many things. Try again.

>> 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].
> 
There are almost 7,000,000,000 people on the planet. We have
not had anywhere near 14,000 people killed by asteroids, I
think their calculation is off.

> 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.
> 
This is, of course, assuming many things:

	1.	Everyone follows the same random ULA allocation algorithm.
	2.	The algorithm is not flawed and yields relatively smooth
		distribution without significant hot-spots.
	3.	People are not lazy
	4.	People read instructions

Assumption 1 depends on assumptions 3 and 4.
Assumption 2 is still relatively unknown as we don't have enough operational
experience with it.
Assumption 3 is pretty well provably false.
Assumption 4 is virtually guaranteed to fail.

Since Assumptions 3 and 4 are non-starters, assumption 1 is seriously flawed
at best.


Owen





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