Internet down but Comcast gateway pingable?

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I have a network connected to a Comcast business modem for internet connectivity. The internet access from my network will go down intermittently for a couple minutes at a time, about every 5-10 minutes. Specifically, what I mean by internet access is the ability to ping an outside IP (so DNS is not an issue).

What I noticed is while internet access is down and I cannot ping an outside IP, I can still ping Comcast's gateway. Wouldn't this mean that my network is not at fault, and that Comcast's servers are having issues?
 
pinging an external ip does not prove DNS is working- ping an external domain name and having it resolve to an IP address would indicate DNS is working.
I'd try a tracert to some public sites and see how far it goes- if it leaves your network, It is probably your ISP.

You also may as well connect a computer directly to the modem after restarting it and test for connectivity, since Tech Support will make you do it anyways.
 
It isnt always DNS that goes down just because you can ping out.

Sometimes in super large networks routers fail to converge properly causing temporary loss of pathway. That is where human beings, i.e. Network Engineers come into the picture and correct it.

During these times you may be able to ping your gateway IP, i.e. the router on the other end of yours, but your traffic may not be able to traverse past that. The best thing to do when experiencing these outages is to perform a trace route to an IP way out there somewhere and see where it stops...

Or go outside and exercise for a bit.
 
/usr/home said:
By gateway, do you mean the gateway of your wan ip?
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Yes

RocketTech said:
pinging an external ip does not prove DNS is working
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I'm not trying to prove anything about the DNS. I'm taking DNS out of the equation. Whether or not DNS works, it will not affect my ability to ping an IP address.

tangoseal said:
It isnt always DNS that goes down just because you can ping out.

Sometimes in super large networks routers fail to converge properly causing temporary loss of pathway. That is where human beings, i.e. Network Engineers come into the picture and correct it.

During these times you may be able to ping your gateway IP, i.e. the router on the other end of yours, but your traffic may not be able to traverse past that. The best thing to do when experiencing these outages is to perform a trace route to an IP way out there somewhere and see where it stops...

Or go outside and exercise for a bit.
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Again, DNS is not an issue. I'm simply not testing DNS because I don't need it for pinging. The rest of what you said, however, confirms my conclusion so far.
 
When it's working find some external traceroute and trace back to your IP, look for the name of the last hop before your IP. If the first 3 letters are ten then it's a Cisco 10k, if it's cdn then it's an Arris Cadant, and if it's bsr, then I'm sorry (it's a motorola BSR which suck). Also take note of the IP of this device. For troubleshooting on your end the type of device on their end doesn't really matter it's just for a bit of social engineering on whoever you talk to when you call. Someone who calls in and says "I can't ping out" is probably going to get the run around at first with the whole "reset your modem, plug a computer directly into the modem, do 20 other idiotic things that you've either already done or know can't possibly be the problem" bullshit, but when you say "Yeah I can ping my gateway IP but I can't ping the 10k/cadant/bsr", as soon as they hear that you know what kind of device they have on their end they'll imediately think you know about 10000x more than the average caller. Another good tool is ping plotter if you want to overwhelm them with data they can't so easily dismiss. Though it sounds like you have a business account so you should get less of a run around anyway.

Use the IP for your CMTS (the 10k/cdn/bsr) and run a continual ping to it and to your gateway IP (or use ping plotter to run a constant trace to say comcast.net or comcast.com with logging enabled), if the gateway pings and you start timing out on the CMTS then it's most likely a problem with your node. I'm about 99.99% sure this will be what you see because if you can ping the CMTS but are still having internet issues then that means the problem is past the CMTS which means 10000+ people would be having this issue and it would have been fixed by now. If possible talk to others in your area that have comcast and see if they're experiencing similar issues, if not it may be a wiring problem on premise for you. Another easy way to test this is to move the modem to a different jack if possible and see if the situation gets better. I'll admit that a while ago I was having problems with my modem and packet loss and such, I figured it was a problem with the node in my area so I called a network ops center in my area and pretended I was a field tech trying to get some modem levels in the area, turns out all the levels were good but mine, so I moved my modem to a different jack and haven't had problems since.

If all that fails definitely call them and provide any sort of logging you can, they hate logs because it shows that you're harder to BS.

But I swear if you send them a traceroute or ping plot that traces just fine to the last hop (or at least to some gateway near the last hop) but has a few dropped packets on previous hops without corresponding packet loss beyond said hops I will personally hunt you down and kick you squar in the nuts.
 
OP I have comcast also and this happens all the time. Im frankly getting tired of it because I subscribe to business class services.

it isnt DNS, it is some other shit I dont know. Im not a cable network engineer. I log into my SMC cable modem device and look at the cable signals and notice that the downstream is not locked and has no signaling during these outages.

Anyone know what the cause is. Dragon nice write up. I am interested to know what Comcast has ...
 
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I used to work as a cable network engineer.

As I was trying to say in the post above yours if it's chronic it's most certainly a problem with the HFC connection between you and the cable company, if it was past that the number of affected customers increases by 2 or 3 orders of magnitude and those problems get fixed quick. It's also most certainly an RF problem as the fiber part typically either works or doesn't, despite the fact that fiber network cables can oft times need cleaning or reseating, fiber going into nodes isn't exactly designed with quick disconnects, it's packed, sealed and screwed down tight. As for who's responsible for the crappy RF that tends to be a bit of a tossup.

Also for business account CSRs come from an entirely different department than normal account CSRs and while normal CSR metrics are "how many customers did you blow off" and "how many account credits did you avoid giving out" business CSRs are more rated on "how many customers did you not lose to other ISPs by being unhelpful", so they tend to be a lot more helpful. Call them every time you have trouble, squeeky wheel and all that. Also be sure to ask "Can you check the levels in my area", cable companies have the ability to poll modems and get all that good stuff like DS signal level and SNR and US transmit power, but they can also look at the devices on the other and and see stuff like DS transmit power and US signal and SNR, they can also tell if it's just your modem offline or others in your area. The reason you care if it's others in the area or just you because if it's just you, it might actually BE you with the problem. If it's not just you then complain away, tell them to fix their shit. If it IS just you then schedule a truck roll anyway just to cover your bases. While you wait for the repair guy there ARE several other things you can try:

Change out any coax cables that you can
Add a bi-directional (or active return) amp
Move the cable modem to a differenet cable jack on the premise.
Either get a really long ethernet cable and an extension cable or a really long coax cable and a coax F-F adapter and either hook up the cable modem outside directly to the cable companies cable that connects to the splitter going into your building, or connect the F-F adapter and run a single long cable into the building directly to the modem bypassing the splitter and any in wall cabling.

If any of those fixes your issue it's bad cabling on your end, if you're hooked directly up to the cable companies cable and still losing sync, it's definitely on their end (either modem or bad plant).
 
Either get a really long ethernet cable and an extension cable or a really long coax cable and a coax F-F adapter and either hook up the cable modem outside directly to the cable companies cable that connects to the splitter going into your building, or connect the F-F adapter and run a single long cable into the building directly to the modem bypassing the splitter and any in wall cabling.--- Dragon


Im gonna do this for sure.
 
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