With the new 2601:5:C80:A0::/60 PD prefix, my outbound IPv6 connectivity terminated at the DIR655's LAN, but I could do pings to IPv6 hosts on the Internet from the DIR655's internal ping test page. Connected PCs were receiving an IPv6 address in the 2601:5:C80:A1::/64 range, but they could not get traffic past the router's LAN. The DIR655 did however respond to IPv6 pings on its LAN using both the local and Internet IP addresses. The router's IPv4 WAN address did not change, and IPv4 connectivity was not effected.
I tried everything I could think of to get IPv6 connectivity back again:
Disabling and re-enabling the DIR655's IPv6 firewall made no difference.
Factory default reset on the DIR655, and minimal manual config didn't help.
Factory default reset on the DIR655, reverting the firmware to the 2.03 level mentioned in the Comcast documentation at »mydeviceinfo.comcast.net/?homega ··· egateway
followed by minimal manual config didn't help either.
I also tried using Comcast's 6to4 tunnel instead of native IPv6, with similar negative results.
All of the above things were tried with the DIR655 isolated from my network with only a single notebook PC attached, so there was no interaction with anything on my network. During this time, I reverted my network to use my old Netgear WNR1000v2-VC, and neither it nor devices behind it had problems with native IPv6 connectivity.
Finally, in desperation, I cloned the DIR655's WAN MAC with the MAC address from a retired D-Link DVG5102s VoIP ATA (because I have seen MAC cloning fix weird Comcast network/routing problems). After doing that, my IPv4 IP address changed as expected (previously that had not changed), and my network IPv6 PD prefix changed to 2601:5:C80:84::/64 as shown below.
With the cloned MAC address, everything works as expected (and I was also to upgrade back to the latest 2.10 firmware release), but if I change back to the original factory MAC address (which had previously been working just fine), the IPv6 problem returns.
This post is primarily just a "heads up" to anyone who might suddenly find their IPv6 connection no longer works (even though the parameters seem to be correct). I am not necessarily looking for help, since I consider the cloned MAC address to be a "fix" (although it should not be required). I will however work with any Comcast tech who wants to do a postmortem to find out why it is happening.
FWIW, while the culprit might seem to be the "/60" PD assignment, I previously have received "/60" PD assignments on this router (it is totally random whether I get a "/60" or a "/64"...it happens with no changes on my end) and did not experience any problems because of it.
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