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Modem randomly reboots

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drjulian
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Modem randomly reboots

Here is our setup:
We have static ip addresses and the modem is in the bridged mode.
We have only 3 devices on the network:
1. Computer A with a web server and a dns server running Windows 10 pro.
2. Computer B with a web server, dns secondary server, and an email server running Windows 10 pro.
3. A netgear wireless router providing wireless to customers

Last week we moved from DSL to Comcast cable. Since the cable was installed we have an issue where the modem will reboot randomly.
Sometimes two or three times an hour, other times it may go several hours before rebooting.
We never had this issue with the DSL and the computers we had with DSL have not changed.

Comcast has changed the modem (Cisco DPC3941B) and tested the line thoroughly. Our upstream and downstream levels are perfect and they have not fluxuated for the entire time.
Our building has several other comcast customers and they have monitored them and us at the same time and none of them go down when ours reboots.

After ruling out the cable and modem as an issue, the next thing we did is look at our machines.

We removed the netgear wireless router but the reboot still occured.
Next, leaving the wireless router off, we removed Computer A, and soon the reboot again occured.
Next, leaving both the wireless router and Comptuer A off the network, we removed Computer B and waited. After 2 hours there was no reboot.

Based on this we conclued that Computer B is having an issue with the modem.
On Machine B is we shut down the Web server, DNS server, and Mail server the reboots seem to stop (we give 5 hours for a test as this is the longest we have gone without reboot since the beginning)

We then try turning on the mail server, and soon a reboot.
We disable the mail server and turn on the DNS server and soon a reboot.
We disable the DNS server and turn on the email server and soon a reboot.

No matter which server runs (alone) the reboots begin again.

We have scanned the machine for virus, malware and rootkit (as we do very often) and it is clean.

We have disconnected the ethernet and connected via wireless to the modem with the same result, so a faulty nic card has been ruled out.

Comcast has sent techs out 6 times in the 8 days we have had their service and they cannot find the problem.
One tech suggested "maybe it is an attack", so we set the network monitor on and waited for an attack and the network usage was unchanged immediately before a reboot.
Also it seems unlikely that an attack would reboot the modem, the modem logs only show the modem getting its address after it powers up again.
No other info is in the modem logs.

Can anyone suggest ideas on how to isolate this problem? Again the same equipment functioned over DSL without any issues for years.

Although comcast seems to be doing all they can (tech sat in my office for several hours Saturday morning as we monitored and tried different things unsecussfully), we obviously cannot functions with random 5 minute drops throughout the day. We process our credit cards through Computer B and when it goes down we are out of business until it comes back up.

 

 

Accepted Solution

Re: Modem randomly reboots

We are encountering the same issue (random reboots) with a very similar setup.

Comcast has replaced the modem three times, has moved the outside cable, etc.

 

We have replaced our network equipment (switch, SonicWall, etc.) but the problem persists.

We're an IT company who provides network services & support to our customers, so we know what we're doing network-wise. (not bragging, just stating that we do this for a living)

 

When the modem reboots, no other equipment is affected.

Up until yesterday's massive Business Phone outage, our phones weren't affected by the Internet gateway reboots.

 

Since we use a VOIP system & VPN to connect to our other offices, this brings everything to standstill until the modem boots again.

 

One Comcast technician said he diagnosed something similar and that the "Maintenance Dept" had finally tracked the problem to the increase in X1 boxes because they send out a signal on the same frequency used to trigger modem reboots.  However, no one else at Comcast appears to believe this.

 

This has been going on for a few months but everything was rock-solid before that.

 

John -Z-

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Accepted Solution

Re: Modem randomly reboots

We finally resolved this, however the solution may not be doable for everyone.

After over 160 reboots in 14 days, I asked the tech if he could get me another modem but NOT a Cisco DPC3941B. He went to their warehouse and got me a old no long used SMC modem, and since he did that, we have not had a single reboot in over 26 hours.

As a test, he left the Cisco DPC3941B installed and ran them in parallel over night and the Cisco DPC3941B rebooted while the SMC did not.

 

So our problem was resolved by getting rid of the cisco modem completely.

 

I am not sure if everyone will be albe to get the tech to dig up an old SMC modem, but if you can, it just might eliminate the reboots.

 

Hope this helps someone, even though we were ultimately unable to determine what was/is causing the cisco modem to reboot.

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12 REPLIES
Trusted Forum Contributor

Re: Modem randomly reboots

Hello drjulian and welcome,

 

You cannot have your DOC3941B (CG Comcast Gateway) in True Bridge Mode (TBM) while utilizing your static IP addresses. When your CG is in TBM its total routing is disabled including your static IP routing. Therefore, any device that is using a static IP address or not cannot performing any consistent routing to support your static IP devices ( Computer A, B or your network router).

 

If you have a 5 block static IP and using one for each of your 3 devices, simply contact Comcast 800-391-3000, use High Speed Internet Technical option, and have the technical agent log into your CG and turn off TBM. As long as each of your static IP device(s) WAN interfaces are programmed correctly you should be able to consistently run all your static IP devices like all other Comcast customers.

 

Hope this helps you out.

drjulian
Visitor

Re: Modem randomly reboots

Thank you for your reply.

 

You are correct that the modem cannot be in TBM. This is my first experience with cable and I may have used the wrong term (before we had DSL and the modem was bridged). I beleive the tech actually called it "pass though".

In checking the modem settings Bridge Mode is in fact disabled and the devices using the static ip addresses are  properly reaching the internet, so routing is not the issue.

 

Having said that, is there anything else you might suggest i can check that could be causing the modem to randomly reboot. On average it is rebooting betwen 10 and 12 times per day, sometimes after 10 mintues sometimes after a few hours.

Trusted Forum Contributor

Re: Modem randomly reboots

If you are sure that your CG is in passthrough, not TBM, then this should not be the problem. Some other troubleshooting issues are as follows:

 

1. If you are not using any of the CG wifi, and you are using your .3) wireless router via one static IP address, log into the CG and disable your LAN DHCP Server. This could be causing DHCP Server conflicts between the CG and your .3).

 

2. Make sure you can log into the internet and can ping and receive replies from all the static IP addresses used for a.), b.) and 3.). Also make sure you can ping your CG static IP address because this is a means by which you can make sure your CG static IP gateway is programmed correctly, and all your a.), b.),, 3.). I have seen other customers resolve intermittent reboot by their CG static IP address not being internally programmed correectly and you might want to have a Comcast technical agent reprogram this for you to make absolutely sure.

 

 

drjulian
Visitor

Re: Modem randomly reboots

Thank you for your reply.

 

As you suggested, we did disable wireless and dhcp on the CG with no change.

We are able to ping all of our static ips as well as the gateway.

Comcast did check the modem configuration and even changed our static ip address to ensure this was not the cause.

 

Any other suggestions?

BHHSREPadmin
New Member

Re: Modem randomly reboots

drjulian and VBSSP-RICH,

So glad to find this post; we have been struggling with our Comcast Modem (DPC3941B) for months!  It randomly(?) reboots 0 to 8 times a day.  I have printed out and analyzed the Event Log looking for any patterns and find none; reboots happen at all times of the day and night (including weekends).

 

We have had Comcast replace it three times.  Same behavior.

 

We do not use the wi-fi capabilities so that is disabled.  

 

Comcast technicians have been out many times for many hours doing many tests. No one seems to have been able to zero in on the cause.

 

Whenever the modem reboots, we lose all Internet connectivity until it comes back up (only a few minutes) and everybody in our building gets upset (since I am the IT guy they all call me!).

I will watch this topic in the hope of finding an answer.

 

BHHSREPadmin

jzupancic
New Member

Re: Modem randomly reboots

We are encountering the same issue (random reboots) with a very similar setup.

Comcast has replaced the modem three times, has moved the outside cable, etc.

 

We have replaced our network equipment (switch, SonicWall, etc.) but the problem persists.

We're an IT company who provides network services & support to our customers, so we know what we're doing network-wise. (not bragging, just stating that we do this for a living)

 

When the modem reboots, no other equipment is affected.

Up until yesterday's massive Business Phone outage, our phones weren't affected by the Internet gateway reboots.

 

Since we use a VOIP system & VPN to connect to our other offices, this brings everything to standstill until the modem boots again.

 

One Comcast technician said he diagnosed something similar and that the "Maintenance Dept" had finally tracked the problem to the increase in X1 boxes because they send out a signal on the same frequency used to trigger modem reboots.  However, no one else at Comcast appears to believe this.

 

This has been going on for a few months but everything was rock-solid before that.

 

John -Z-

BHHSREPadmin
New Member

Re: Modem randomly reboots

All,

 

Each morning I log into the GUI of the modem and check the Event Log to see how many times the modem has rebooted since yesterday.  It is almost NEVER zero times ... and as many as eight (in one day).  The event log has entries like this:

2016/7/14 01:26:44  Notice [Dhcpc][1678]: erouter0 got new IP address xxx.xxx.xxx.xxx (address is always the same).

 

Does anyone know what the number after "Dhcpc" means and is it important?

 

Is there anything else I can do to help troubleshoot why this is happening?  

 

Seeking resolution,

BHHSREPadmin

drjulian
Visitor

Re: Modem randomly reboots

We finally resolved this, however the solution may not be doable for everyone.

After over 160 reboots in 14 days, I asked the tech if he could get me another modem but NOT a Cisco DPC3941B. He went to their warehouse and got me a old no long used SMC modem, and since he did that, we have not had a single reboot in over 26 hours.

As a test, he left the Cisco DPC3941B installed and ran them in parallel over night and the Cisco DPC3941B rebooted while the SMC did not.

 

So our problem was resolved by getting rid of the cisco modem completely.

 

I am not sure if everyone will be albe to get the tech to dig up an old SMC modem, but if you can, it just might eliminate the reboots.

 

Hope this helps someone, even though we were ultimately unable to determine what was/is causing the cisco modem to reboot.

BHHSREPadmin
New Member

Re: Modem randomly reboots

drjulian

Odd, but that is where we are right now too.  After replacing the Cisco DPC3941b twice - with no change in the frequency of reboots - we will "downgrade" to the older SMC modem (the one we had before the upgrade).

 

Question: when you did your test, with both modems connected to Comcast, and the Cisco unit still rebooted but the SMC did not, was there anything connected to the Cisco (on your side)?  If the only connection (besides power, of course) was the coaxial from Comcast, then there is no way your network could have been causing the Cisco to reboot, yes?

 

The only reason we changed to the more capable Cisco modem was we signed up for a more robust package with almost twice the bandwidth.  To my knowledge the Cisco DPC3941b is the only modem Comcast offers for the more robust service.  Am I mistaken?

 

BHHSadmin

drjulian
Visitor

Re: Modem randomly reboots

When we did the test with both modems, our network was connected to the SMC. The Cisco had nothing connected (save power and coax from Comcast). So, yes, it was clearly not our network that was triggering the reboot.

 

I cannot speak to the packages that Comcast offers, but our package is 25 down /10 up and the SMC is providing 30/11 right now so we are satisfied.

 

One point. When the SMC was first installed it did not acheive the correct down/up speeds. In fact, we had only 2 upload channels (vs the 3 we had with the cisco). We contacted tech support and requested a firmware upgrade. I assume they did it because overnight the SMC logged one reboot and the next day our speeds were fine and we had 3 upload channels. So after you install the SMC it would be a good idea to request the firmware update.(The update may have been automatic; I was never able to confirm that Comcast tech support initiated it).

Johnacook
New Member

Re: Modem randomly reboots

Thanks for this thread. I have been having this issue for years it seems and have had every device Comcast provides. I too have tried several of the cisco routers and they rebooted multiple times a day and the other issue with the cisco router is that it takes several minutes to come back online. I had them get me a Netgear and its better, but still randomly reboots at least once a day. Any ideas? I assume it's the load from my network but I still don't think this is acceptable. I have dozens of devices behind the device.

Thanks.

Re: Modem randomly reboots

I'm having this exact same issue. Its nice to find this thread and know I am not crazy. One jerk supervisor last week told me "Your cleaning lady must be power cycling the modem." What?!?!?! I'm definitely complaining about that guy.

 

I've had a Cisco modem for 2 months. And it takes almost 3 minutes for the modem to come back online after a reboot. Previously to that we had a Netgear which would come back in about 1.5 minutes but obviously still had the issue. Today they are bringing us an SMC. I'll be sure to check my upload speeds before they leave.

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