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Re: draft-ietf-v6ops-addcon-03.txt ... ULAs of shorter-than-/48 and ULA multicast scope matching ...

Hi,

On Mon, 12 Mar 2007 23:43:44 +0000
Tim Chown <tjc@ecs.soton.ac.uk> wrote:

> On Fri, Mar 09, 2007 at 01:02:00PM -0500, John Spence wrote:
> > 
> > Comment #1:
> > 
> > >> How about something like:
> > 
> > "ULAs are, per the specification, normally /48 prefixes.  Note that a
> > larger organization with, for example, a /44 allocation, that wanted to
> > be able to map ULAs one-to-one with global prefixes, could generate 16
> > ULAs.  These would not summarize, however.  It seems reasonable that the
> > organization could also simply generate a /44 ULA, by using fewer than
> > 40 bits (36 bits in this example) of the "Global ID" specified in RFC
> > 4193."
> >

I'm slightly confused be this, and I think the comment about not
being able to summarise 16 /48 ULAs is what is confusing me.
Summarisation seems to me to only be useful if you are going to
announce a summary to an upstream entity of some sort, and is really
only for the benefit of the upstream entity (e.g. upstream ISP, IGP
backbone area, other areas carrying inter-area routes rather than a
default etc.), as it reduces their route table and other routing
resource requirements.

As ULAs are kept within private entity, and therefore there isn't any
upstream entity, is there really any need to summarise to less than
16 /48 ULAs (in your scenario, for example)? If, within your
organisation, your routers can't cope with 16 /48 ULAs, I'd think you
problably have bigger routing problems to deal with (and a default
route only might be a main (and only) solution to that)!

If you were summarising ULAs internally to scale an IGP by dividing the
IGP into areas, would 16 areas, corresponding to the 16 /48s (global
and ULAs), be too many to cope with ? I'm not really sure they would.

I do see the operational benefits of having the /44 to /64 bits in the
global and ULA addresses match. I'm not quite sure I see how
summarisation by itself, which is how I read your paragraph, would be a
justification to do that, when the consequences are increased chances
of collision, should you join your /44 ULA domain with somebody else's.

Have I somehow misunderstood what you're trying to get at?

Regards,
Mark.