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Jason609's profile

Contributor

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13 Messages

Tue, Feb 23, 2021 6:42 PM

DPC3941B Firmware Upgrade Broke IPV6

Comcast pushed a firmware upgrade to my DPC3941B modem last night and broke my IPV6 connectivity. I had asked for help on these forums previously and Tier 2 Support rolled back my firmware back to fix the issue, this was fine until last night. Additionally, the modem has not been stable since the upgrade and keeps rebooting and I can no longer access the modem's internal web server through static gateway IP address. Can you assist with a firmware roll back or device upgrade to fix these issues? 

New Contributor

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5 Messages

1 y ago

I hate to "me too," but I'm also experiencing a lot of what's been discussed in the thread. Heck, if I was to give a simplified explanation of what I'm running into, it would look almost identical to the comment "sfsmcipv6" made.

Speaking of the web interface, here's what I've discovered:

- If I connect to the DPC3941B's IPv4 address from inside my network ("true static IP" subnet), it doesn't respond on port 80.

- If I plug a laptop directly into the DPC3941B, let it give me a 10.1.10.x DHCP address, it responds to my laptop just fine (at 10.1.10.1) on port 80.

- But... If I try to connect to the DPC3941B's IPv6 address from inside my network (an address that's obviously hard to remember/type), it actually does respond just fine on port 80!  (so I bookmarked this, to avoid having to run into the network closet with my laptop until they fix this nonsense)

Of course none of this addresses the more frustrating main issue, which is that it'll suddenly refuse to pass traffic from the DHCPv6-PD delegated prefixes assigned to my internal subnets, for no reason, until I reboot the modem.

P.S. If anyone wants to compare firmware versions, here's what I dug out of the web UI: Boot version: 3.4.22.4, Core version: 3.12.14, Download version: 2.3.10.13_5.5.0.5

New problem solver

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10 Messages

1 y ago

My 3941B appears to have received another firmware push this morning that fixed my v6 route delegation issues.  My only subsequent change was to NOT use Prefix ID 0, as it gave me the same /64 as on the subnet connected to the modem.  Changing that to anything else provides a working delegation.

I also found that the modem appears to prefer https://10.1.10.1/ now, and it also seems to only work on the 10.1.10.x/24 subnet.  I just added a 10.1.10 address to my firewall and blocked NAT for the destination address of 10.1.10.1.  Yes, I also believe it's v6 address will work on port 80, but I've had lots of problems trying to use it as it continually logs me out.

My current code versions are:

BOOT Version: 3.4.22.1
Core Version: 3.12.14
Download Version: 2.3.10.13_5.5.0.5

New Contributor

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5 Messages

1 y ago

Looks like you're on the same firmware version as me (if I'm reading the right numbers).  Are you sure the update fixed the issue, or was it just the reboot that happened as part of the update? (In which case, the issue may return for you in a few days.)

Also, now that I've tried the web interface via IPv6 a little more, I've also noticed that it doesn't work quite right. (Seems to want me to login again every time I navigate to a different page.) I may do the same firewall trick you did to make it easier to get into it at some point.

New problem solver

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10 Messages

1 y ago

Update absolutely fixed it.  I fought with this thing quite a bit, including multiple power cycles of the modem, and could never get it to work correctly.

Yes, we shall see if this works for longer than 6-8 hours...

New Contributor

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8 Messages

1 y ago

It's fixed for me too. At least it's been working for a bit more than 24 hours.

I can confirm that talking to port 80 form a 10.1.10/24 IP also seems to work.

My version numbers are the same as derek_konigsberg's, except for BOOT which is 3.4.22.4.

Thanks all!

New Contributor

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5 Messages

1 y ago

Well, it broke for me again. The usual, where all of the sudden IPv6 traffic from DHCPv6-PD delegated subnets stops getting forwarded out by the cable modem.  I know if I power-cycle the box it'll start working again (perhaps for a few days), but I'm kinda sick and tired of this.

Official Employee

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41 Messages

Hi Derek Konigsberg, Our team can help. When you're ready, please click on the secure link below to connect with a specialist. You may need to disable any pop-up blockers to start your chat. 
https://comca.st/3bUalK8

New problem solver

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10 Messages

1 y ago

Broke for me last night the exact same way Derek.  Bounced the cable modem to fix it.

Testing the setup behind a Technicolor GCA4131 at another location, been working for ~2 days so far.  If that is more stable, I may ask to get this Cisco swapped out.

New Contributor

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5 Messages

1 y ago

I should probably mention that I did some rudimentary testing with ping and tcpdump, and noticed something peculiar... When IPv6 breaks like this, inbound IPv6 packets actually do still get through! Its just the outbound ones that never seem to go anywhere. So I guess its only partially broken. (Suspecting a firewall bug in the cable modem, but that's just a wild guess right now.)

New Contributor

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8 Messages

1 y ago

It broke for me too only a couple of hours after I posted that everything was working. I knew I spoke too soon. Interesting that the inbound keeps working.

Official Employee

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28 Messages

@Sfsmcipv6, I am sorry to hear that the problem continues to be ongoing. We would be happy to investigate this further for you. Can you send our team a private message with your full name, business name, and business address? To send a private message, click the chat icon in the top right corner. Choose "Comcast Business" as the handle when sending the message. We look forward to hearing from you!

Contributor

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21 Messages

1 y ago

I suspected the modem's firewall too.

If you look at the modem's firewall logs you'll see a suspiciously large number of dropped packets being recorded after IPv6 stops working.  I never found a way to reset just the firewall though so I wrote a script to reboot the modem when the network goes down.  Unfortunately it went from breaking roughly every 2 days to breaking every 2 hours (with no changes in my local configuration).

Contributor

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12 Messages

1 y ago

This seems to be happening even after applying the Ubiquiti config early in the thread. Seattle, still broken.

New problem solver

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10 Messages

1 y ago

Works for maybe 24 hours but then breaks again.  /56 delegations still in place and I see v6 traffic heading toward the modem but there is never return traffic.  Traceroutes go to the modem LAN interface and stop.

Firewall logs show 43k+ IPv6 forwarding drops today in spite of having completely disabled the v6 firewall and rebooted the modem a few days ago.

Contributor

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21 Messages

1 y ago

I ended up switching to AT&T Business Fiber as a result of the ongoing Comcast IPv6 prefix delegation issues.  I spent way too many hours over half a year trying to workaround the instability of Comcast's equipment including modem swaps, talking to customer service, reading forums, writing scripts to automatically reboot stuff when it breaks, learning more about IPv6 than I ever cared to know, dumping packets, and a lot of head scratching.  Finally enough was enough.

AT&T's implementation of IPv6 prefix delegation isn't perfect: their modem only hands out /64s but it routes packets reliably and you can request up to 8 prefixes with some careful configuration using multiple DHCPv6 clients.  At least there's no need for hacky workarounds like NDP proxies or periodic reboots.

My set up has been stable and reliable for several days now so I consider the experiment a success.  Plus it cost about the same amount of money for 5x the connection speed.  The installation process was a bit painful (it took two weeks to get an appointment) but the crew was professional and did a great job making everything tidy just like I wanted.

I'm slightly worried that AT&T won't guarantee that my IPv6 address range will remain static but it has sometimes changed with Comcast too (after modem swaps) so it's not a big loss.  I have yet to feel the renumbering pain again and if it ever becomes a problem then I can likely automate my way out of it.

I wish Comcast would take technical issues with their supplied equipment more seriously.  These IPv6 prefix delegation routing problems seem like they should be pretty easy to diagnose and resolve if they would only engage their engineering staff to investigate them.  Until that happens, I certainly won't be recommending Comcast's services.  It's 2021; IPv6 should just work!

Anyhow, this was my experience.  I hope it helps.  I know switching ISPs isn't always an option but it might be worth considering for lack of any other breakthroughs.  Maintaining a half broken network is no fun at all.  Good luck!

(edited)

New Contributor

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5 Messages

1 y ago

The most frustrating part of all of this is that the root cause of most of these issues is that they simply provision the cheapest CPE (customer premise equipment) they can possibly buy that meets their paper specs. If I was allowed to buy my own CPE, and still keep my service /w static IPs, I'd immediately go and do the exact opposite: Buy the most expensive, reliable, high-end DOCSIS modem/router I could get my hands on.

One person in these forums apparently managed to do that, but the process relied on techs giving him certain routing protocol passwords that they'll never admit are available to us... and can change on us at any time.

I feel like IPv6 has been this "on again, off again" experiment I've been dabbling in for like 20 years now. It really should be something that's given just as much priority to work correctly as IPv4 by now.

Contributor

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12 Messages

1 y ago

This is so strange.
I wish I could learn more about what the INPUT DROPs were.