Help with DHCPv6 client-identifiers
Peter Grandi
pg_dhcp at dhcp.for.sabi.co.UK
Sun Nov 20 15:37:46 UTC 2011
>>> 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.
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