I'm on a business account because I needed five static v4 addresses. Trying to get PD working again.
I'm still getting a single /64 for devices connected directly to the Comcast router, but when I view its "LAN ipv6 setup" page, it says the "ipv6 prefix" is :: instead of the /56 that used to be assigned, and which the router used to give out a /60 via dhcpv6 PD.
In case it's helpful, the network is set up with the Comcast router plugged into the cable connection, then on the private side there are two devices: one Cisco SRX family VPN box for work, and one custom Linux machine with two NICs in it. It runs a dhcp server on its internal NIC, and dibbler-client on the external one to get the PD prefix. (So three levels of routing.)
The router is a CG3000DCR. No switches between it and either the SRX or custom Linux machine, except the switch inside the router.
This had been working for several weeks after we replaced the Comcast router with one that had v6 not-intentionally-disabled (see the "ipv6 firmware rollback" thread). But it seems to have died at some point in the last three weeks, and I just now got time to investigate.
...And apparently the problem was that the router just lost its mind at some point. Checking my account admin page showed the v6 /56 prefix that I expected, and then rebooting the router picked it back up.
Sigh. I guess the old windows 95 solutions are still the best sometimes. Should have tried that earlier.