IPv6 routing on DPC3941B in pass through mode

Hi! Thank you for your patience and for reaching out to Comcast over social media. I am sorry to learn about this experience and that we have made you feel this way! I will certainly pass along your feedback so we can make the corrections necessary to prevent this kind of unfavorable experience from happening in the future. What I would like to do from here is also schedule a technician appointment and request for our field team to see if we can get you a Cisco modem. In order to further assist, please send us a private message with your name, address, and account number or a phone number listed on the account. 

I am experiencing the same problem.  My modem is CGA4131COM.

I cannot get anyone to do anything to resolve the problem.  I have been trying to reach someone that will put out more effort than sending a reset signal to my modem for a week.  I have not received a phone call back from tier 2.

If you can help me I would really appreciate it @Comcast_Gabe

Good luck with getting it fixed. Comcast Gabe just told me it was working as expected. I guess not working is expected. I'm currently working with my second provider and a tunnel provider. Researching a 3rd provider to provide dual stack service for me. I'm paying way too much money each month for not getting support.

You won't get the response that you are hoping for.

As stated, IPv6 other than a single /64 directly connected to the Comcast CPE is best effort, i.e., you're not guaranteed anything but that (and even that is poorly supported). I have not renewed my business contract, nor do I plan to as I will almost certainly have fiber to my location by spring, and I know for sure it'l be a cheaper MRC as well as have functional dual stack.

I would consider renewing this as a backup at a lower tier, maybe, if it provided more than my current cellular backup for less money.   

Any address within the entire /56 works via directly connected interface to the modem. That bypasses my entire security stack, which is unacceptable. DHCP-PD is supposed to be routing the assigned prefix to my router, instead the next-hop is sending neighbor solicitations on the interface looking for everything I send its way when return traffic comes back. The comcast next-hop router thinks that everything is locally connected instead of routed via the host it handed the DHCP-PD prefix delegation to. This was all working for me, then they did an upgrade and it stopped. Since then it hasn't been resolved. Tier 2 support just replaced the modem and closed the case. Same thing is happening as was before the modem replacement. Taking 4 hours out of my day so they could replace the modem and get everything working again as it was with IPv4, just to prove it wasn't the modem, and then just assume it was corrected is unacceptable. Multiple emails to the Tier 2 support engineer go unanswered, just decided he was done and that is that. Mention that here and the staff just state that is how it is supposed to work, which it is not.

I tasted this exact pain for months.

I gave up, it's broken, they know it. It is incredibly low priority for them - translated: they care very little about it because it's .01% of their customer base that notice and care. Last I heard it was known to the modem firmware team but not even accepted as an actual problem to work on. 

I have been told in no uncertain terms that this won't be fixed on certain modem models, and fixing it to work as we expect (read: correctly as a delegation) is best effort at most. I encourage you to go through the process with Comcast, but be warned, it's incredibly demoralizing and likely won't yield you anything but pain. On the up chance it gets you action, I want to know about it.  

Already wasted those 15 days of my life with Tier 2 support. It was working, then all of a sudden they decided to push a firmware upgrade. Now no IPv6 and a random sub-5 minute outage periodically. So much for customer satisfaction.

I'm sorry to hear about all the trouble you are having with the routing. I'd be glad to ensure that this gets looked into further. Please click on my handle (Comcast_Gina) and send a private message with your name, the business name, the complete service address (including city, state, ZIP, suite number, etc), and the phone or account number, and any pertinent details-- like the ticket number(s)-- so we can help. 

We'd be glad to work on this routing issue for you, it sounds like you've already done a lot to get it figured out. If you'd like us to help,

please click on my handle (Comcast_Gina) and send a private message with your name, the business name, the complete service address (including city, state, ZIP, suite number, etc), and the phone or account number, and any pertinent details we can use to help you out. 

Comcast confirmed their lack of delegation support here: https://forums.businesshelp.comcast.com/t5/IPV6/IPv6-Delegations-DHCP-PD/td-p/42516

If you need delegation... find another provider. Comcast does not support or care to support it. No matter how many posts there are on the forum, it just does not matter.

Hi, there! Thanks for spending your time today to reach out to Comcast over Forums! I am sorry to learn that we have made you feel this way. We are doing our very best to respond as quickly as possible during these challenging times. We are very grateful for your patience in waiting for a response. Prefix delegation is described as beyond our line of demarcation as internal networks vary so drastically from business to business. We will certainly do all that we can to help with our equipment or wiring. We can assist by making requested changes, however, most of these settings fall into the realm of your local network and are beyond our demarcation of support.  If the requested changes do not work properly, we recommend that you work with local IT personal for further local network troubleshooting and support. Here is also a great link to some details on how to get the IPv6 working on a Cisco DPC3941B modem. Please let me know if this helps. https://comca.st/2TQsyOZ

Everything was working for me. A firmware update was made on your modem, and everything stopped working. A replacement modem made no change. Troubleshooting the issue with Tier 2 support provided no help at all. My own troubleshooting showed that you are sending neighbor soliticitations for all addresses instead of routing the traffic to me. Having a single subnet of a /56 is not proper. I've shared my account information multiple times, I've spoken with multiple support individuals. It still doesn't work, and it is a misconfiguration on the comcast side. DHCP-PD is broken on the comcast network, some one from Tier 3 or higher support needs to figure out that issue. I've shared multiple packet captures support to show what is happening, but I don't think anyone has actually looked at them. Traffic leaves my network, and the comcast network starts doing neighbor solicitations when it comes back.

We'd like to get some more information so we can help. Please send us a private message so we can get to work on this. 

Anything other than one /64 directly connected to the Comcast business gateway is officially unsupported. I have been told by multiple level 2 techs. They know it is broken, and it's such a small percentage of their customer base that even knows that it is functionally zero-level priority.