DPC3941B Firmware Upgrade Broke IPV6

Wanted to provide an update since it may be helpful to others. Since I couldn't get any help with my DPC3941B and needed to get my internet working reliably again phone support had it swapped with a CBR-T CGA4131COM. Soooo, IPV6 PD does not work when using my router. It will work for a single /64 if connected to the LAN of the CBR-T but not for setups anymore complicated than that. So basically still have a /56 that I can't use!

I went through this same issue over a year ago and Comcast still has not been able to fix the issue with IPV6 when the CBR-T needs to route. Has anyone at Comcast Business sent this issue to the CPE department? It's pretty crazy that this issue is documented all over these forums and elsewhere and still there are no clear explanations from Comcast as to why this can't be fixed. 

Welcome to the club.

My working hypothesis is that as helpful as customer service would like to be, they don't actually have a way to escalate issues to a network engineer to replicate, diagnose, or address the underlying problems with their equipment.

At this point, I would take my business elsewhere but there isn't anywhere to take it to.

Another Update. So I managed to get IPV6 PD working with my Ubiquiti EdgeRouter 4. After I committed the following I had to unplug the ethernet connection to the CBR-T and reconnect it, then IPV6 started to work, confirmed on test-ipv6.com. The following lines were added to my configuration:

Note: Interface eth1.10 is the LAN, change this to yours. Change Prefix-id for each vlan (if applicable) e.g. :1, :2, :3 and so on. I will update if this continues to work long term.

 dhcpv6-pd {
     pd 0 {
         interface eth1.10 {
             host-address ::1
             prefix-id :1
             service slaac
         }
         prefix-length /59
     }
     rapid-commit disable
 }

just confirming this. see my reply to "/59 prefix delegation block changed" for details. Please post if anybody is getting any worthwhile support to fix this (not just from the Comcast_* bots).

I'll give you some more info which may be helpful. My modem was changed from the BWG (DPC3941B) to the CBR-T (CGA4131COM), my /56 IPv6 block remained the same. However, when IPv6 PD worked on my BWG (for about a year) the first /59 assignment was e0. A /56 contains 8 /59s, so e0 would be the 7th out of 8 of this block, the 1st is used by the BWG. Not sure how it came up with this but it remained the same for the time I had the BWG. Now with the CBR-T, the 1st /59 is for the modem and then it will PD the next /59 blocks so I get 20, 40... etc. I think it is a difference in how the firmware in the modem does PD. Unfortunately, I doesn't seem like there is currently a way in the CBR-T to "statically" bind these /59 PD to the downstream device.

@Jason609 :  This is my exact problem.  There are a few of us in the thread I started on /59 changing.  I also had a "later" /59 assigned for quite some time & all of a sudden it changed back to the first one & broke routing.  Feel free to join us in the "/59 prefix delegation block changed" thread if you'd like.

I can report the same issue here, also with the DPC3941B, unfortunately with no solution yet.

I'm glad to see the community getting some traction better understanding these connectivity issues.  I hope they'll be resolved for real someday.  It's 2021 after all and IPv6 really should just work out of the box.

As for me, I've booked an installation appointment for AT&T Business Fiber.  The service offers Static IPs, officially claims to support IPv6 with prefix delegation, costs a bit less than Comcast, offers faster speeds, and is available on a month-to-month basis.  I'm wary of trading one set of problems for another but I'm cautiously optimistic.

Good luck everyone!

@Comcast_Marcos, your handle doesn't pop up in the folks I can Peer-To-Peer with, though there is a @Comcast_Marc.

@amessina : Try to send a private message to "Comcast Business" and that will get the right person in touch with you.

I also have a DPC3941B and am experiencing this - IPv6 PD works at reboot but then it stops routing IPv6 traffic after ~8-12 hours. Further, the web interface to interact with the modem is never responsive, even right after reboot. Connections to port 80 time out. Started a few days ago. Can't confirm whether it's correlated with a firmware update since the user interface is not reachable.

@sfsmcipv6 similar to my experience. Looks like having my modem replaced fixed the issue for now (stable IPv6 for 24 hrs now). The prefix is still "random" on reboot, but clearing the DUID a couple times will eventually lead to the old prefix. 

I believe the modem I had was a "Cisco" modem (vertical modem, 4 ethernet ports) and they replace it with a "Technicolor" modem (horizontal, 8 ethernet ports)

@flybynight thanks for your reply. your technicolor is working with prefix delegation? do you have static IPv4 IPs? from reading other posts on this forum I it seemed like the technicolor modem was still broken with prefix delegation.

I do have a static /29 for IPv4. So far, IPv6 works just like it worked with the old modem prior to the firmware upgrade. I get a "random" /59 out of the static /56 assigned to the modem. But now, once I got that /59, it appears to remain the same as long as my DUID doesn't change. So certainly an improvement.

Looks like there may be different "Technicolor" modems. Mine is the CGA4131 with 8 Ethernet and 8 Voice ports. It is a horizontal modem (just a bit too tall for 1U ;-) ).