I've been going back and forth with ZyXEL Tier 2 Tech Support for two months and no solution yet. Their engineers overseas (Taiwan?) looked at the problem and reported back that they were able to get IPv6 to work with their ISPs.
Just to be clear, I'm trying to get ZyXEL's Pure IPv6 implementation to work with Comcast as described in the USG 200's User Guide (the instructions in the User Guides for Pure IPv6 and Prefix Delegation IPv6 are identical across USG line - 50/100/200/300, etc.).
So I've followed the instructions exactly as stated in the guide for Pure IPv6 and I also tried Prefix Delegation. Neither work. After configuring, the Xfinity IPv6 Readiness Test gives me a score of 0 out of 10.
None of ZyXEL's people in California have Comcast as an ISP so they haven't been able to test the USG series to see if they can configure it to work with Comcast's IPv6. I was told they all have either Verizon FiOS, Time Warner, or AT&T U-Verse.
It should work, but the problem is a knowledge gap in trying to get it configured properly for Pure IPv6 (I'm not interested in nor do I care about 6-to-4 tunneling).
The good news is they, ZyXEL, are now in contact with a Comcast engineer and are sending them a USG to try to determine what settings need to be changed in order to get it to work. They'll let me know if they find a solution.
That's all I have for now.
I should post in the BBR Comcast Forum to see if anyone has been able to successfully get a USG to work with Comcast's IPv6. But I'm too tired at the moment and it's probably a long shot anyway.--