DHCPv6 PD & Routing Questions

Mark Andrews marka at isc.org
Wed Nov 25 23:59:50 UTC 2015

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.

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

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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