IPv6: numbering of point-to-point-links

Lasse Jarlskov laja at telenor.dk
Tue Jan 25 08:44:34 CST 2011

Thank you all for your comments - it appears that there is no consensus
on how this should be done.
Thank you for the reference to this draft, Marco - and to Ron as well.

Both RFC3627 and this draft appears to be stating that these issues a
E.g. whether an implementation will perform subnet-router-anycast on a
small subnet, whether Neighbor-Discovery-messages are rate-limited etc.

Does anyone have any information on how different vendors handle this -
even in older software?

/126 appears to be the middle path - which is reserving the
subnet-router-anycast address while keeping the subnet as small as
possible. This appears to address both issues - at least for Ethernet
(one free address will hardly constitute a potential Neighbor Cache
Exhaustion issue).

Best regards,

-----Oprindelig meddelelse-----
Fra: Marco Hogewoning [mailto:mch-nanog at xs4all.nl] 
Sendt: 24. januar 2011 14:11
Til: Lasse Jarlskov
Cc: nanog at nanog.org
Emne: Re: IPv6: numbering of point-to-point-links

> While reading up on IPv6, I've seen numerous places that subnets are
> all /64.
> I have even read that subnets defined as /127 are considered harmful.

RFC3627, with a lot of discussion in the IETF on this. See also

> However while implementing IPv6 in our network, I've encountered
> of our peering partners using /127 or /126 for point-to-point links.

I personally don't any benefit in using /126 subnets.

> What is the Best Current Practice for this - if there is any?
> Would you recommend me to use /64, /126 or /127? 
> What are the pros and cons?

>From an operational point of view there is a risk that be using /64
somebody can eat away a lot of memory by either scanning or even
changing addresses. This is also described in the draft above...

I would personally recommend to at least always assign the /64, even if
you would decide to configure the /127. RFC 3627 has been around long
enough that you will keep running into equipment or software that won't
like the /127. In which case you can always revert back to /64.
This will also allow you to use easy to remember addresses like ::1 and
::2, saving you the headache of a lot of binary counting.



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