Help with DHCPv6 client-identifiers

[Peter Grandi]

>>> I'm pretty sure you're allowed to use a /127 for point to
>>> point links.

>> Uhm, I think that "works" but I suspect it is not necessarily
>> standards compliant.

> Sure it is: http://tools.ietf.org/html/rfc6164

No, it is not legal for point to point links in general, it is
only legal for *inter-router* links (which were already sort-of
not totally discouraged by previous RFCs):

 "Scope of This Memo

  This document is applicable to cases where operators assign
  specific addresses on inter-router point-to-point links and do
  not rely on link-local addresses."

 "For the purposes of this document, an inter-router point-to-point
  link is a link to which only two routers and no hosts are
  attached.  This may include Ethernet links that are configured
  to be point-to- point.  Links between a router and a host, or
  links to which both routers and hosts are attached, are out of
  scope of this document."

And this is a very recent (April 2011) exception made explictly to
override previous RFCs and with the narrowest of special casing:

 "[RFC3627] discourages the use of 127-bit prefix lengths due to
  conflicts with the Subnet-Router anycast addresses, while stating
  that the utility of Subnet-Router anycast for point-to-point links is
  questionable.

  [RFC5375] also says the usage of 127-bit prefix lengths is not valid
  and should be strongly discouraged, but the stated reason for doing
  this is to be in compliance with [RFC3627].

  Though the analyses in the RFCs are correct, operational experience

  with IPv6 has shown that /127 prefixes can be used successfully."

 "Routers MUST support the assignment of /127 prefixes on point-to-
  point inter-router links.  Routers MUST disable Subnet-Router
  anycast for the prefix when /127 prefixes are used.

  When assigning and using any /127 prefixes, the following
  considerations apply. Some addresses have special meanings, in
  particular addresses corresponding to reserved anycast addresses. [
  ... ]"

This is legalized cheating :-).

It does not help for example the original poster and his DHCP
problem which is about VMs each on their own VLAN (IIRC, and I
destest VLANs anyhow), because those VMs are not routers.

So while most likely using a /127 in the original poster's case
with PREFIX::1/127 and PREFIX::2/127 will work, it is strictly
speaking not covered by this.