Linux DHCPv6 server for PD on ppp interfaces

I don't know the answer to your question, but I just want to warn you about using the DHCP server address as your default router. If that works it is by chance... In IPv6 the default route is communicated by RA's and not by DHCP.

You will need to enable forwarding, accepting RA's and sending out RS's. A problem here is that the kernel included in Centos6 does not allow you to do that. When forwarding is enabled then accepting RA's and sending RS's is disabled. There should be a CentosPlus kernel with the relevant patches though. See the mailing list thread / message at http://lists.centos.org/pipermail/centos/2011-April/thread.html#109756

The relevant sysctl.conf settings are:

net.ipv6.conf.all.forwarding=1
net.ipv6.conf.eth0.accept_ra=2
net.ipv6.conf.eth0.accept_ra_defrtr=1
net.ipv6.conf.eth0.router_solicitations=1

I've got a debian Linux box doing almost the same thing (I don't use a dhcpv6 server and I'm using dibbler as the ipv6 dhcp client for the ppp interface), in my PPP config I've got:

defaultroute
replacedefaultroute
+ipv6
ipv6cp-accept-local

if you add:

debug

to the ppp config you'll get the details of the ipv6 negotiation. I suspect you're missing the defaultroute option