/127 between routers?

[Benedikt Stockebrand]

Hello Steinar and list,

sthaug at nethelp.no writes:

>> But anything using addresses from the existing ranges may well assume
>> a /64 prefix in full compliance with the standards.
>
> Anything assuming only /64 prefixes in the existing ranges would most
> likely fail spectacularly given the number non /64 prefixes in use.

absolutely correct, but the only way to gain from a fight with a
vendor of a /64-only product is to stay out of it beforehand---it
doesn't matter whose "fault" it is if something doesn't work.
Customers have an understandable tendency to go with vendors and
service providers that they consider trouble-free.

As we have already seen, RFC 4291 is somewhat misleading as far as
section 2.5.1 is concerned---one has to read beyond that section to
learn about the /64-only issue.  So this is likely to cause trouble in
the future even if somebody took pity right away and updated the RFC
with some clarifications.

The easiest way to stay out of this kind of trouble is by adhering
strictly to the standard while not expecting anybody else to do so.
With a "gain" of not doing so being in many cases "saving" a
sub-percent fraction of an abundant resource it's not exactly a major
investment, at least if you decide to do so when you first start to
design your addressing scheme.

> For instance, we number our router loopbacks with /128 prefixes from
> our global /32. Similarly, our links use /124 prefixes. We have no
> intention of changing these to use /64 only.

Sure, as long as it works for you.  I am certainly not telling you to
change an existing network design, especially when I don't know
anything about the environment you are working in.

>From my particular background however I'd just be afraid that such a
setup eventually blows up in my face the moment I'd least expect it.
Security updates to transit routers or home routers that assume /64
subnets and for some unrelated reason hit the market big time
immediately come to mind.  Call me paranoid, but I've seen these sorts
of things happen.

And, having worked for a large ISP catering primarily for private end
users, I actually see a valid reason for home routers *not* to support
anything but /64 subnets: If your customers can't misconfigure the
subnet prefix, that eliminates yet another potential problem.  If
things work, customers are happy; if things don't work, they'll blame
you (after all, that router works with your competitor's service) and
call your support.  So to an end user ISP a "feature" to configure
arbitrary subnet prefixes just costs you money and reputation.


Cheers,

    Benedikt

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