Point 2 point IPs between ASes

Thomas Bellman bellman at nsc.liu.se
Fri Jun 30 07:40:30 CST 2017

Hash: SHA1

On 2017-06-29/17 17:06, Job Snijders wrote:

> On Wed, Jun 28, 2017 at 11:09:25PM +0200, Thomas Bellman wrote:

>> I know that many devices allow you to configure any subnet size, but
>> is there any RFC allowing you to use e.g. /124 or /126?

> Breaking the law! Some IETFers will come hunt you do, be aware! ;-)

:-)  But I figure if the standards disallow certain things, then there
will likely be implementations that don't handle those things.  You
might need or want to interoperate with those, and when it is you that
is not following the standards, you can't complain much to the other

Of course, for a point-to-point link, it tends to be fairly easy to
check with the other end what they support.  And if they (or you)
change equipment, then you will have a downtime for that link anyway
and can change your addresses at the same time.  Tends to be less
easy for a network with multiple hosts.

> Here is some historical perspective looking at the IETF standarsd and
> current Internet-Drafts:
> RFC 3513 "only /64 is valid"
> RFC 3627 "don't use /127, use /126 if you must"
> RFC 4291 "reaffirming: only /64 is valid"
> RFC 6164 "a /127 is OK to use too"
> RFC 6583 "there are problems with /64"
> RFC 7421 "/64 is the best!"
> RFC 7608 "every prefix length must be forward-able"
> RFC 4291bis-07 "fine, /64 and /127 are valid, but nothing else!"
> draft-bourbaki-6man-classless-ipv6-00 "IPv6 is classless FFS"
> RFC 4291bis-08 "fine, /64 and /127 are valid, and anything defined in future standards, and anything configured manually"
> Quoting from 4291bis-08: 
> """
>     Interface Identifiers are 64 bit long except if the first three bits
>     of the address are 000, or when the addresses are manually
>     configured, or by exceptions defined in standards track documents.
>     The rationale for using 64 bit Interface Identifiers can be found in
>     [RFC7421]. An example of a standards track exception is [RFC6164]
>     that standardises 127 bit prefixes on inter-router point-to-point
>     links.
> 	Note: In the case of manual configuration, the Prefix and    
> 	Interface Identifier can be any length as long as they add up to    
> 	128.
> """
>     source: https://tools.ietf.org/rfcdiff?url2=draft-ietf-6man-rfc4291bis-08.txt
>     full file: https://tools.ietf.org/html/draft-ietf-6man-rfc4291bis-08

Thanks for this information!

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