IPV6
Internet Protocol Version 6
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IPv6 routing on DPC3941B in pass through mode

We have a DPC3941B in pass through mode with static IPv4 and IPv6 addresses. I am having trouble getting IPv6 routing through the modem to our firewall.

 

There are three possible ways that this could work:

 

1. Put the modem in bridge mode and lose our statics. This will allow our Ubiquiti router to use DHCP6-PD straight from Comcast. We've had success with this in the past, but I'm hesitant to do this because we'd like to use our statics.

 

2. Keep the modem in pass-through mode, feed the modem its WAN and LAN address ranges using DHCP6-PD (this happens), and then our Ubiquiti device uses DHCP6-PD against the DHCP service on the Comcast modem to get its WAN and LAN addresses (this does not happen). We have the stateful DHCP6 option enabled in the modem, but that looks to be standard DHCP6 for clients using the modem as the router, not a DHCP6-PD server that would further delegate ranges to our firewall.

 

3. Use static routing. We assign our Ubiquiti WAN and LAN addresses within our delegated /56, and then configure a static route in the modem so that it knows where to send the traffic. This would be a great solution, but I can't see any place in the modem to configure IPv6 static routes.

 

What is the official method for getting usable IPv6 in pass through mode with statics? Do we really need to choose either static addresses or IPv6?

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Occasional Visitor

Re: IPv6 routing on DPC3941B in pass through mode

The issue has finally been resolved. On 12/13 I noted that it was a routing issue that would be "painful to resolve", and it was indeed painful. It took over a month, with over 50 DMs between me and the social media support team here, 2 unnecessary modem swaps, and over half a dozen tickets escalated to tier 2 and closed without them ever contacting me or even looking at the issue.

 

It was frustrating, but at the same time understandable. We have lived in a world where NAT has been the standard for about 25 years, so even for the tier 2 people IP routing isn't a thing: give the modem a WAN address, the customer handles the LAN side, done. Our issue was that we were being assigned a delegated prefix on the LAN side that was clearly not being routed, but the tier 2 guys would ping the WAN address, get a response, and close the ticket. Even if you could get them to understand that the WAN wasn't the problem, they're programmed to respond that if it's on the LAN side it's outside of their demarcation even when it's publically routable address space assigned by Comcast.

 

To get it resolved I finally had to escalate the issue to the executive support team, who managed to get it resolved in about 4 business days. Pretty reasonable response time for an issue this complicated, if we could have skipped the month of wasted time before that.

 

I wanted to provide actionable information for future customers who find this thread, but unfortunately I don't think I can. It will always be painful dealing with a bureaucratic utility when your problem can't be resolved by checking the noise on the line or rebooting the modem, but the silver lining is that this support case is being used to improve the documentation and playbooks used by the tier 2 teams to diagnose and escalate issues. In short, it sounds like there might be some "the LAN is actually meaningful when using IPv6" training coming soon that will make this experience smoother as IPv6 becomes more commonplace.

 

So, thank you to the support team that has been very patient with my begging and pleading to get the right people on this issue, and I'm happy to finally resolve this thread.

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Community Manager
Community Manager

Re: IPv6 routing on DPC3941B in pass through mode

Hey holmosapien (love the handle LOL)! Thank you for reaching out about the IPv6 routing! Most businesses have their networks set up a bit differently based on their needs but this forum thread has some good information that may be useful here, although the modem model is not the same one: Can not get internal IPv6 traffic to route with the CGA4131COM . If none of that helps, please click on our handle (ComcastBiz_Support) 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 about the current network setup so that we can take a closer look into this and help out Smiley Happy 

 

-Gina

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Re: IPv6 routing on DPC3941B in pass through mode

That thread suggests that you can't have both statics AND IPv6, since the modem must be in bridge mode for DHCPv6-PD to work. I'll send you a DM, since maybe there are additional options only available to you guys.

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Re: IPv6 routing on DPC3941B in pass through mode

Perfect, we will wait for your response in a private message so we can look into this for you.

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Re: IPv6 routing on DPC3941B in pass through mode

I haven't heard anything back, and apparently there's a 1 private message limit.

 

Is there a way you guys can configure a static IPv6 route on our modem so that we can route some space within our delegated prefix to our firewall? I can handle the firewall configuration, I just need a route on the modem.

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Re: IPv6 routing on DPC3941B in pass through mode

Hi there. I'm so sorry for the delay. Believe me, your time is very important to us and we want to take care of this for you. We will definitely look into this for you. To be safe, I am going to work with our sister teams to get you the correct information, and set this up for you if possible. We will reach back out to you in the morning. Thanks so much for your time. We will talk to you soon. -Rob K.
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Re: IPv6 routing on DPC3941B in pass through mode

The key to getting the DPC3941B working in pass-through mode is to turn off the "rapid-commit" option in the DHCPv6-PD client on the Ubiquiti firewall.

 

Rapid commit condenses the DHCP exchange from four messages (solicit, advertise, request, reply) to two messages (solicit, reply). Turns out the DPC3941B will only do prefix delegation if your DHCP client performs the full four-message exchange. With rapid commit, the modem will only give you a WAN address and no delegation.

 

Now we're having a problem where our delegated prefix isn't being routed -- no traffic destined for addresses within our delegated prefix make it through our modem. Traffic from our LAN makes it to its destination, the destination replies, but the reply is lost before it gets back to the WAN side of our firewall. I suspect this issue will be painful to resolve, but we're making progress.

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Re: IPv6 routing on DPC3941B in pass through mode

Hey there! I hope you are having a nice Friday evening! I just wanted to follow up with you today to see how everything is working. In reviewing the dispatched ticket on the account, I am seeing that our Advanced Repair team was able to resolve the service issue.

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Re: IPv6 routing on DPC3941B in pass through mode

Nope, after 17 days our delegated prefix is still not being routed. Pretty ridiculous.

 

If I could get someone on the phone I would love to help troubleshoot the problem, but so far everyone has been completely unwilling to communicate with me about this issue.

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Re: IPv6 routing on DPC3941B in pass through mode

I am sorry to learn that your delegated prefix is still not being routed after 17 days. I would feel the same way if I experienced a delayed resolution. Would you mind confirming if our Advance Repair team were able to contact you and provide you with any additional information?

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Re: IPv6 routing on DPC3941B in pass through mode

No, they have not contacted me at all. I called today at noon to check on the status of the ticket, and they promised me a response within 4 hours. I did not receive any calls.

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Re: IPv6 routing on DPC3941B in pass through mode

The issue has finally been resolved. On 12/13 I noted that it was a routing issue that would be "painful to resolve", and it was indeed painful. It took over a month, with over 50 DMs between me and the social media support team here, 2 unnecessary modem swaps, and over half a dozen tickets escalated to tier 2 and closed without them ever contacting me or even looking at the issue.

 

It was frustrating, but at the same time understandable. We have lived in a world where NAT has been the standard for about 25 years, so even for the tier 2 people IP routing isn't a thing: give the modem a WAN address, the customer handles the LAN side, done. Our issue was that we were being assigned a delegated prefix on the LAN side that was clearly not being routed, but the tier 2 guys would ping the WAN address, get a response, and close the ticket. Even if you could get them to understand that the WAN wasn't the problem, they're programmed to respond that if it's on the LAN side it's outside of their demarcation even when it's publically routable address space assigned by Comcast.

 

To get it resolved I finally had to escalate the issue to the executive support team, who managed to get it resolved in about 4 business days. Pretty reasonable response time for an issue this complicated, if we could have skipped the month of wasted time before that.

 

I wanted to provide actionable information for future customers who find this thread, but unfortunately I don't think I can. It will always be painful dealing with a bureaucratic utility when your problem can't be resolved by checking the noise on the line or rebooting the modem, but the silver lining is that this support case is being used to improve the documentation and playbooks used by the tier 2 teams to diagnose and escalate issues. In short, it sounds like there might be some "the LAN is actually meaningful when using IPv6" training coming soon that will make this experience smoother as IPv6 becomes more commonplace.

 

So, thank you to the support team that has been very patient with my begging and pleading to get the right people on this issue, and I'm happy to finally resolve this thread.

View solution in original post

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Official Employee

Re: IPv6 routing on DPC3941B in pass through mode

Wow, we are so glad to see that everything is now resolved. We really do appreciate your patience with us and if you do ever need anything, please let us know as we are your one-stop-shop 24/7! 

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Re: IPv6 routing on DPC3941B in pass through mode

What was your fix? I am on my 3rd Tier 2 ticket with it being closed after ping'ing the WAN IPv6 address.

 

Same boat... the delegated /59's are not being routed.

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Official Employee

Re: IPv6 routing on DPC3941B in pass through mode

I am very sorry to hear that you are having an issue with your IPV6 routing. I would love to assist you. Can you please reach out through private message with your first and last name, full service address and account number or phone number so we can assist you in getting to the bottom of this? 

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Re: IPv6 routing on DPC3941B in pass through mode

I am trying to do DHCPv6-PD with a DPC3941B and I believe I have exactly the same issue.  I am able to get a /59 from the cablemodem, but it attempts to do ND to find the hosts, instead of routing through my gateway router.  You can ping my router just fine, because it's on the same LAN as the cablemodem.  But for anything behind it, it's "destination unreachable" because that route is missing.

 

Is there any way you can give a hint to what the resolution was?

 

I have called support (actually National in my case) and they understand the issue, and they are continuing to help me; but they don't yet know how to resolve the issue.  It would be enormously helpful if there were any kind of hint about what was able to make things work.

 

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Re: IPv6 routing on DPC3941B in pass through mode

I am having the same issue. I have statics and my IPv6 was working fine until recently.

 

I suspect that comcast broke the DPC3941B's ipv6 with a firmware upgrade similar to theCGA4131COM. I was hoping comcast would come up with a solution by now since this has been a know issue for some time.

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Re: IPv6 routing on DPC3941B in pass through mode

I appreciate you reaching out to us through our business forums today and I am very sorry for the delayed response. I would like to assist with the IPV6 routing issues. Can you please reach out through private message with your first and last name, full service address and account number or phone number and we will get to the bottom of this? 

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Re: IPv6 routing on DPC3941B in pass through mode

It seems holmosapien is not willing, or allowed, to share the eventual solution with us. It is great to hear better training materials will be provided, but as long as things don't actually get fix that is useless.

 

Today I ran into this problem. I had a NetGear CDR 3000, but along with a contract (also going from 75 -> 100) renewal was forced into a modem replacement (I did not want it, but was forced). Swapped the modem, waited for firmware updates and static IP confiration while tech waited. Confirmed all that was working, but IPV6 was not!

 

I plug my EdgeRouter into the modem's LAN port and the EdgeRouters WAN side is configured with one of my static IPs. IPv4 works fine. Investigaation with packet sniffing confirms the very same problem. Prefixes are received, but not properly routed by Comcast Technicolor CGA4131COM. It performs a neighbor solicitation instead. Nothing was changed on my LAN equipment, only the modem swap. Bleh!

 

So either there is an actual fix, or an equipment swap is needed to a model modem that handles this correctly. No matter what Comcast says, if it works correctly on the NetGear and on some other Comcast supplied devices but not the Technicolor CGA4131COM than that device is at fault and its firmware should be fixed!

 

I'm going to need a real fix from Comcast, or holmosapien if willing/able (DM is fine too), or an equpment swap. I have seen reports that the DPC3941B works, although it may introduce unwanted etra latency...

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Official Employee

Re: IPv6 routing on DPC3941B in pass through mode

Thank you for your patience and for providing this information. We truly want to help and do everything we can to provide a fix for the IPV6 routing issues. Please feel free to send us a private message to further assist.

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Re: IPv6 routing on DPC3941B in pass through mode

Keep in mind that I haven't been to the office in over three months so I don't know the current state of IPv6 support on the CGA4131COM, but when I was fighting all of this there were two issues at play:

 

1) Our modem could get an IPv6 WAN address that worked, but the delegated prefix was not being routed. This was not related to the modem, and happened with every modem Comcast gave us. This was an upstream network problem, and the only reason it took us over a month to get it fixed is because nobody at Comcast that we normal people are allowed to talk to understands layer 3 routing. They have a script that tells them to ping the WAN address, and when they can they give up and say "must be on your side!". I finally was able to escalate it to a different group that was allowed to talk to real network engineers who fixed the broken route.

 

2) Once the routing problem was fixed, I found that the CGA4131COM has broken firmware that, at least a few months ago, will not route IPv6 if you have a static IPv4 assignment. You can find Comcast representatives saying in this forum that it's a known issue and they would have a fix by the middle of November (2019!), but months later I couldn't even get the executive support guys to tell me what the status was. The only way I could get it to work was by putting the modem in bridge mode, which bypasses the IPv4 static. That frustrated our marketing guys because they liked being able to filter our IP address from our website statistics, but it was a compromise we could deal with.

 

I just SSH'd into a computer at the office and I notice that as of right now IPv6 is broken there, too. It could mean that our IT guy took the modem out of bridge mode, or it could mean that there's been a firmware update that has broken my site just like yours. I will tinker around with it more this coming week and see if I can come up with a working configuration.

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Official Employee

Re: IPv6 routing on DPC3941B in pass through mode

Thanks so much for taking the time to reach out to us through our business forums. I very much appreciate your patience and greatly apologize for the delay in our response. I know how important the services are to your business and we want to do all that we can to assist. I am sorry to hear that you are having issues again with your static IP's. I can certainly review the modems settings for you and work on a resolution. Can you please reach out through private message with your first and last name, business service address and account number or phone number? 

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Re: IPv6 routing on DPC3941B in pass through mode

I have a /56 static subnet for ipv6, and I believe I am having the same routing problem with the DPC3941B as well. I've been suspecting it's either a route issue with the modem, or an upstream route issue as well.

 

A linux router/firewall device is connected to the modem that's in pass-through, and it can auto-configure and route a /64 subnet on the external interface just fine. I can ping out to any ipv6 address from the router without a problem.

 

I have multiple vlans on internal interfaces that I can assign additional /64 subnets on, and these are delegated out to clients just fine. However, none of these devices are able to communicate past the router.

 

The internal devices can ping the router's external IP address without a problem, but they can't ping the modem's IPv6 address on the same /64 subnet as the router's external interface.  The router can ping the modem's address without any problems.

 

I've been able to use IPv6 with a Hurricane Electric tunnel for quite some time, using the same routing/subnet structure without a problem, but the moment I try to use the native IPv6 I encounter this routing issue. 

 

 

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Official Employee

Re: IPv6 routing on DPC3941B in pass through mode

Hello, how are you? I hope overall your day is going well. Thanks for all the details about what you are experiencing using your service and third-party equipment. I need to make sure everything looks good with your modem and service. Are you able to send a private message so I can take a detailed look at your account? If you can include your name, service address, and account number this would be perfect. I can use the phone number listed on the account instead of the account number if that's easier. 

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