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[Connectivity] Uploads only working on 1 laptop in the house

1) Livermore, CA

2) 1 laptop in the house works with uploads - my desktop, phones, work laptop do not - they all get downloads of 35mb/s but when I try doing anything that requires a bit of uploading - it times out, and we lose internet access until I stop what ever I was trying to upload.

the one laptop that works was the first laptop to be plugged into the router/modem and activation. This is my 3rd modem/router from Comcast. The first two were the latest models but the said they were faulty so they swapped it out for a netgear router and an Arris Modem.

I can connect to Skype, upload small documents, and other small upload tasks OK (not great, super slow) but anything like a speed test, uploading a video, or video calls will crash the network.

3) Issue is constant.

4) Arris Modem
Netgear router
All on latest firmware

5) Router is about 10 feet away from devices

6) No firewalls set up, and I disabled any anti virus for the tests thinking that could be it.

I have done the restarts of the modems/routers/computers - I'm not quite sure what to do next. Of course, Comcast support has a thing for restarting modems 6 times in a call so I am avoiding them.

thanks for any help!


2 edits
Marc, has this behavior remained constant across all three modem/router deployments or is it new to this current Arris/Netgear setup?

Is behavior same when multiple devices are cabled into the router (and radio off) as when all are connected wirelessly? I'm assuming everything has been wireless but please confirm.

Please post modem & router model numbers.

Since you just joined today, be aware that BBR partners with ISPs - including Comcast - to offer members direct access to what has proved to be a higher caliber Tech support. Responses are slow but they stick with you until it's fixed and no PR chitchat BS like must be suffered via phone. After years of one guy from Comcast, Steve, being our dreamboat tech he has moved along and Comcast has assigned two new people. Not saying you should go that route but it's an option.

It's over here: »Comcast Direct


The behavior has not been consistent - the reasons for the modem switch was that after a week, my downloads wouldn't go higher than .5mb/s - uploads were non existent. Second modem (same model) they gave me was also bad. This set up works flawlessly with downloads, and uploads only really work on the laptop I used to set up and activate the network which is why I feel it's a DNS/IP address issue.

Modem: CM820
Router: Wireless N 100 WNR1000v2

I just hooked up one of the problem device to the router directly and the issue was resolved for that particular device which is interesting so I guess in this case plugging in directly once will fix the issue? Is this something common? I am a software engineer but I do not work with networking that much - I just moved out of my parents house for the first time where I've had internet set up for me my whole life haha.


Palmyra, VA
reply to MarcMaiden
Are you using QoS ?


Thanks for all the help,

I'm not sure how to tell if I'm using QoS - I didn't see it in the router settings

EDIT - doesn't look like this model comes with QoS

Ok - here is some more odd behavior

It looks like my router is only letting one device have upload capabilities at a time - It's what ever device was plugged in directly to the router last.

The test:
I plugged my work laptop in to the router - I was getting 36mb/s down, 6 up

I took it off the hard connection - same results

I plugged in my girlfriends laptop - I was getting 36 down, 6 up
Work laptop stopped being able to upload
I unplugged my girlfriends laptop- still getting 36 down, 6 up

I put my work laptop back in, was back to getting 36 down, 6 up but my girlfriends laptop couldn't upload anymore.

I've never heard of this type of behavior before...are the computers getting different IP addresses when being plugged in or something?


3 edits
reply to MarcMaiden
shit. I wrote a reply then walked dog before posting. In the interim your new info makes my questions obsolete so deleted.
2am here and need to sleep.

Your new test sounds like a DHCP issue but first post suggested multiple simultaneous devices worked fine *except* if one attempted a sustained upload. That wouldn't be DHCP.

G'luck with it!


Figured it out!

The MTU (Maximum Transmission Unit) was set too high - It was 1500 which should be ok for comcast, but I guess it was sending packets that were too large which was crashing my network - Not sure why the difference, but setting it to 1470 did the trick!

reply to MarcMaiden
So your using a Netgear router that has no QOS whatsoever? Hmmm it's definitely your outdated router. Buy a new router. Netgear has had QOS in all their routers for at least 5-8 years. Something smells here & it's your equipment.

reply to MarcMaiden
said by MarcMaiden:

setting it to 1470 did the trick!

Hmm. That doesn't make sense to me but, hey, you got it working.