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jeffb's profile

Contributor

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21 Messages

Thu, Feb 25, 2021 7:30 AM

Losing connectivity every 2 hours

My modem has started losing IPv6 connectivity every two hours, like clockwork.  It appears to coincide with the WAN DHCP lease expiration.

Prior to this afternoon, my modem was losing IPv6 connectivity every two days, like clockwork.  Still terrible.

This new state of affairs is unbearable.  I effectively have to reboot my modem every two hours to maintain IPv6 connectivity.

I guess a new configuration just got pushed and it's breaking my network something awful.

I'm beyond hope that the modem's broken firmware will be fixed anytime soon (see ample evidence in forum and support tickets).

Can we at least raise the WAN DHCP  timeout to a couple weeks as a workaround?

Anyone else started seeing this issue?

Responses

Official Employee

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21 Messages

2 m ago

Hello and Happy Thursday! I hope you’re having a great week! I also appreciate you taking a moment out of your day to reach out to us on our Business Community Forums! You’ve reached the best place and perfect person to help with virtually all of your account needs! Going forward, I'm truly sorry to hear about the IPv6 packet loss you're experiencing with your services as it's definitely not what we want for you! You mentioned there could be some impact from the WAN DHCP lease expiration, are you able to set up a static IP for this specific port to see if that helps? If not, we can also take a closer look on the backend to see what other options we have to help. Please send us a private message by clicking my name "ComcastJosephW" then select "Send a Message" on the right side with your name and service address to get started. Thanks again for both your time and patience!

Contributor

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21 Messages

2 m ago

I don't control the WAN DHCP expiration.  This is an address that the modem retrieves from Comcast's own network automatically.

Everything within my internal network uses leases that are handed out by my own router behind the modem.  The router itself has a static IP address.  None of this has changed recently.

On further inspection, I am actually losing IPv6 connectivity every hour (not every two hours).  Rebooting the modem restores IPv6 connectivity for an hour and then the modem stops routing traffic again.

So my theory about the problem being related to 2 hour WAN DHCP leases may be incorrect.  (Renewing a lease should not break connectivity but the modem's firmware is buggy enough that anything could be the culprit.)

Regardless, looking at my logs makes it very obvious that something changed upstream around 4:53 pm PST on 2021/2/24 and I have lost IPv6 connectivity every hour since then.

Official Employee

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21 Messages

Ok, thank you for these further details in regard to your network topology as it definitely paints a clearer picture for me although I'm truly sorry to hear about the consistent IPv6 drop outs you're having as I know this can be frustrating to say the least. Especially since they are occurring about every hour for you. We may need to push a new provisioning signal to the gateway itself or even replace the device altogether, however, are you noticing this happening on multiple devices connected to your router or just on one in particular at this time? 

Contributor

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21 Messages

2 m ago

I don't think the hardware is broken.  The Comcast modem's firewall was already dropping IPv6 traffic every couple of days, now it's just happening every hour very predictably.

As for affected devices...

- The modem itself can ping IPv6 hosts on the Internet.  I tested this using the modem's built-in diagnostic features.

- The router behind the modem can ping IPv6 hosts on the Internet.  I tested this using the router's built-in diagnostic features.

- The devices behind the router can ping IPv6 hosts on the Internet, but only for one hour, after which time the modem's firewall drops all inbound IPv6 packets sent to them until I reboot the modem.  I tested this by running pings and connecting to IPv6 sites on those devices.

As far as I can tell, ONLY rebooting the modem fixes the problem (temporarily).  Rebooting the router or devices behind the router has no effect.

This appears to be an issue with the Comcast modem's firewall and its support for IPv6 prefix delegation, as I previously described here: https://forums.businesshelp.comcast.com/conversations/ipv6/ipv6-prefix-delegation-workaround-and-lingering-reliability-issues/5fe0a64fc5375f08cd9aba6d

I spent many hours with customer support on this issue before, got absolutely nowhere, and gave up.

What I don't understand is why as of yesterday the Comcast modem is getting into a bad state in a span of one hour whereas previously it routed IPv6 packets for about two days between reboots.  So now it's pretty unusable.

This problem should be easy for anyone at Comcast to replicate in the lab.  It just takes a modem, a router configured for prefix delegation, a device downstream from the router, and a stopwatch.

Official Employee

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3 Messages

Have you made any changes regarding your modem? 

I had a similar scenario to this one and what was happening was a temporary DHCP was not auto-renewing due to a new router the customer added to their account. 

 

Have you tried to remove and replace the IP? I wanted to make sure you tried this step before going into anything else. Thank you for your in-depth responses, as well. 

Contributor

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21 Messages

2 m ago

I haven't changed anything on my modem, router, or network topology in at least a few weeks.

What do you mean by "remove and replace the IP"?  I have static IPv4 and IPv6 addresses.  As far as I can tell, they're still valid.

Could it be that Comcast remotely pushed a configuration change or firmware update to my modem within the last 48 hours which affected IPv6 routing or firewall behavior?

Official Employee

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21 Messages

Typically provisioning/firmware updates would have to be done manually and wouldn't come as a surprise to you, however, I cannot say for sure without taking a closer look from the backend. When you mention you haven't changed anything with your network topology within at least a few weeks, what equipment changes did you make recently exactly? Are you still using the same router? Also, what happens when you bypass the router and use the gateway for routing functionality, are you still loosing IPv6 packets after running pings for over an hour as well? 

Contributor

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21 Messages

2 m ago

Recent network changes:

- Several months ago, I set up monitoring tools and wrote some scripts to automatically reboot the Comcast modem when it stops routing IPv6 traffic.

- A few weeks ago, I tried modifying the DHCP lease expiration timeout on the Comcast modem to see if it would have an effect on IPv6 routing issues.  It did not have an effect, so I reverted the change.  The modem continued to have IPv6 routing issues every two days or so.

- Yesterday, I noticed that the modem was losing IPv6 routing and being rebooted every hour as of that afternoon.  I posted to this forum but didn't make any changes.

- Today, after seeing that the modem was still losing IPv6 routing every hour, I temporarily disabled my script so it wouldn't reboot the modem anymore because it was too much of a service interruption.  And since I had nothing better to do, I backed up settings, upgraded my router firmware, and performed other routine maintenance.  IPv6 routing is still broken.

Summary: Same modem, same router, same topology, more frequent outages as of yesterday.

I haven't tried bypassing the router.  However, the router itself is able to ping IPv6 hosts even when downstream hosts can't so I suspect that if I plugged a computer into the Comcast modem directly, then it would work.  However, my network topology is complex enough to require a router (multiple vlans, firewall, services) so I can't just plug everything into the modem.

My guess is that devices plugged directly into the Comcast modem are using link-local addressing to reach the gateway and the gateway's routing tables handle link-local routing fine but not prefix delegation to devices further downstream.

When I first reported this issue to Comcast last year, I used a packet sniffer and could see the Comcast modem dropping inbound IPv6 packets instead of forwarding them to the router.

Official Employee

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21 Messages

I appreciate these details and the time you've taken to outline all the updates performed recently. To be honest, the equipment that you decide to build behind our modem/gateway is a bit out of our demarcation point as any settings or firmware updates performed is not something we can necessarily control. If you're able to hardwire or use our gateway's routing functionality directly without any IPv6 or 4 packet loss, then it would indicate to me that there could be settings within the router behind the modem that is the culprit. I definitely understand that you have a bit more a complexity in your topology regarding your setup. To confirm, are you using ethernet switches for you VLANs by chance? You might be able to hardwire these hosts that are experiencing the most packet loss etc. 

Contributor

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21 Messages

2 m ago

No, the router is fine.  Like I said, I ran packet sniffers and I could see that after some period of time the modem simply stops forwarding downstream traffic to the router.

How else would you explain that my IPv6 routing issue goes away when I reboot the modem?  ONLY when I reboot the modem!  Touching the router makes no difference at all.

Similarly, I made no changes to my network at all and then suddenly IPv6 routing issues start happening every hour instead of every two days.  There's only one piece of equipment in my network whose configuration I don't completely control and that's the modem (and whatever Comcast infrastructure it talks to).  How else would you explain the change in frequency of my outages?

So as far as I can tell, the problem isn't beyond the demarcation point, it's right on the demarcation point.  Lots of folks have reported similar problems in this forum.  Isn't it time to stop punting these problems onto customers and instead start investigating their common cause?

Official Employee

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3 Messages

Let's take a look at your account on our side. This will let us confirm the equipment on the account and see how it looks from our backend. There are a couple of questions we would like to ask further, but it regards account info that we can not share through a public forum. Can you please private message me your full name and service address? Thank you for posting to our forums. 
To send a private message, please click my name "ComcastMartin" then select "Send a Message" on the right side.

 

I understand you brought up about speaking with Customer Service already but our team will do our best to get this situated for you.