NANOG 40 agenda posted

Stephen Sprunk stephen at
Sat Jun 2 05:39:49 UTC 2007

Thus spake "Randy Bush" <randy at>
>> the average number of v4 prefixes per AS is ~10, and it's
>> rising.  In v6, the goal is that every PI site can use a single
>> prefix**, meaning the v6 routing table will be at least one (and
>> two or even three eventually) orders of magnitude smaller
>> than the v4 one.
> how much of the v4 prefix count is de-aggregation for te or by
> TWits?

A quick look at this week's CIDR Report says 35.9%, or 78,738 routes.

[Update to earlier stats: The current v4 prefix/AS ratio is 8.7.
However, there are ~11k ASes only announcing a single v4 route, so that 
means the other ~14k ASes are at a v4 ratio of 14.3.  In contrast, the 
current v6 ratio is 1.1 and the deaggregate rate is 1.2%.]

> why won't they do this in v6?

The simplistic answer is that nearly all assigned/allocated blocks will be 
minimum-sized, which means ISPs will be capable of filtering deaggregates if 
they wish.  Some folks have proposed allowing a few extra bits for routes 
with short AS_PATHs to allow TE to extend a few ASes away without impacting 
the entire community.

While many have derided the "classful" nature of IPv6 policies, the above 
was one of the reasons that it's being done.  The other, obviously, is that 
IPv6 is big enough we can do it that way and skip all the administrative 
hassle of worrying about how much space people "need" and focus on whether 
they "need" a routing slot (as much as the RIRs pretend they don't care 
about routability).

I said "simplistic" above because there will be a few extremely large orgs 
that will end up getting larger-than-minimum blocks, and they could 
deaggregate if they want to -- or deaggregate more bits than other folks get 
to.  There's not much that can be done about that (without vendors inventing 
cool new knobs), and I already addressed why it shouldn't be that big a deal 
anyways in the ** footnote you snipped.

However, this relies on RIRs rejecting "but we need to deaggregate" as 
justification for larger-than-minimum blocks.  OTOH, the community may see 
how small the v6 table is and decide that N bits of deaggregation wouldn't 
hurt.  After all, with ~25k ASes today, and router vendors claiming to be 
able to handle 1M+ routes, it seems we could tolerate up to 5 bits of 
deaggregation -- and 3 bits would leave us with a table smaller than v4 has 


Stephen Sprunk      "Those people who think they know everything
CCIE #3723         are a great annoyance to those of us who do."
K5SSS                                             --Isaac Asimov 

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