DHCPv6 PD & Routing Questions

Owen DeLong owen at delong.com
Sun Dec 6 00:05:44 UTC 2015

> On Nov 25, 2015, at 15:59 , Mark Andrews <marka at isc.org> wrote:
> In message <CAMWxDfrh+O=SPZwPmAZhYnvAEeK2eMFw3CD0qf34Fkbb=-SaPw at mail.gmail.com>, Brian Knight writes:
>> On Tue, Nov 24, 2015 at 6:34 PM, Baldur Norddahl
>> <baldur.norddahl at gmail.com> wrote:
>>> DHCPv6-PD allows multiple PD requests. But did anyone actually implement
>>> that? I am not aware of any device that will hand out sub delegations on
>>> one interface, notice that it is out of address space and then go request
>>> more space from the upstream router (*).
>>> DHCPv6-PD allows size hints, but it is often ignored. Also there is no
>>> guidance for what prefix sizes you should ask for. Many CPEs will ask for
>>> /48. If you got a /48 you will give out that /48 and then not honor any
>>> further requests, because only one /48 per site is allowed. If you are an
>>> ISP that gives out /48 and your customers CPE asks for a /56 you will still
>>> ignore his size hint and give him /48.
>> Or, worse, the ISP's DHCPv6 server honors the new request and issues
>> the larger prefix, but refuses to route it.  Ran into that myself when
>> I replaced my home CPE router, and changed the prefix hint to ask for
>> a /60 block (expanded from /64) at the same time.  That made for a
>> frustrating few days without IPv6 service, waiting for my original
>> delegation to expire.  (Tech support, of course, had no clue and
>> blamed my router.)
>> In retrospect I should have perhaps had my original CPE generate a
>> DHCP release message for that prefix before disconnecting it.  But I
>> won't be the last person to fail to generate that.
>> -Brian
> Well the requesting router could announce the route.  ISC's client
> has hooks that allow this to be done.  That is, after all, how
> routing is designed to work.  The DHCP server usually is sitting
> in a data center on the other side of the country with zero ability
> to inject approptiate routes.

Are you really suggesting that a residential ISP accept routes advertised
from their customer’s CPE? Really?

That’s about the most ridiculous thing I’ve heard on NANOG in a long time
and that’s saying something.

> The DHCP relay could also have injected routes but that is a second
> class solution.

Maybe, but in an ISP/Customer PD environment, it’s certainly preferable
to what you consider a “first class” solution.


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