DHCPv6 PD & Routing Questions

Brian Knight ml at knight-networks.com
Wed Nov 25 21:30:12 UTC 2015

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.


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