Using /126 for IPv6 router links
mysidia at gmail.com
Sat Jan 23 17:07:05 CST 2010
On Sat, Jan 23, 2010 at 7:50 AM, Dobbins, Roland <rdobbins at arbor.net> wrote:
> On Jan 23, 2010, at 7:56 PM, Mikael Abrahamsson wrote:
"We should forget about small efficiencies, say about 97% of the time:
premature optimization is the root of all evil" --Donald Knuth
> A couple of points for thought:
> 1. Yes, the IPv6 address space is unimaginably huge. Even so, when every molecule in every soda can in the world has its own IPv6 address in years to come, it might not seem so big.
Then obviously, it's giving every molecule in every soda can an IP
address that is the waste that matters. There are several orders of
magnitude between the number of molecules in a soda can (~65000 times
as many) as the number of additional IPs used by giving a
point-to-point link a /64.
When comparing the number of molecules in a soda can TO 2^64.
It's like in the IPv4 world comparing a /30 to a /31. And
arguing it's wasteful to give a point-to-point link a /30 since
all it needs (in theory) is a /31. Near the beginning of IPv4
(before exhaustion was an issue). when at the same time standard
practice was allocating /13s to users who will use at most a /20
in 10 years.
Optimizing this early creates potential issues and reduces flexibility
The designer/operator should not confuse design patterns that use more
IP addresses than the minimum technically possible, for a block of
addresses, with design selections that are gross wastes of address
space -- such as assigning every molecule its own IP.
IPv6 is a very large address space, so it's LARGE optimizations
that matter, such as not giving every molecule its own IP. Not
small optimizations that matter, such as using a /126 for a
relatively small number of P-t-P links (in the grand scheme of
things) versus a /64.
Anyways, I would suggest reserving a /64 to each P-t-P link, and
(If you prefer) set static neighbor entry for the peer (in the case
of Ethernet) and configuring a /72 (smaller than what you have
reserved). For the sole reason of disabling IPv6 autoconfig
and neighbor discovery.
Technically everything to the right of the /64 boundary is reserved
for the HOST portion, and such is the design of IPv6.
This allows for greater scalability than assigning a longer prefix.
If that specific connection is ever to be replaced one day with a
link that's _not_ point-to-point, or to be expanded, then the
designer has greater flexibility: an option that does not require
re-numbering the changed link.
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