Question Slower internet after upgrading modem, but only when going through router

Apr 8, 2019
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I have a Cox cable 300MB plan and they convinced me to upgrade to a SURFboard SB6190 modem saying I would see 5x speeds of my current older modem. I had been seeing 120MB over my ethernet network. After upgrading the modem and rebooting modem and router a dozen times I am now seeing only 50MB through the enternet, although 300MB when connected directly to the modem. All speed tests are via Speedtest.net.

For testing purposes I disconnected all devices from the network except the one PC, so we are dealing with just the modem, router, and PC below:

MODEM
SURFboard SB6190 (32x8)

ROUTER
Linksys E3200
DD-wrt

PCs
Windows 7

I have swapped the cable between the router and modem a couple of times, but the old cable was getting 120MB so that doesn't seem to be the culprit, and I am using the same cable that achieves 300MB to the computer. Wi-fi speed is only about 25MB although the connection claims over 144MB to the router.

I have scoured this forum and the internet for solutions, but all problems are slightly different so no solutions apply. It seems like it must be a router setting, but I'm not sure how to begin troubleshooting. Any settings to check or strategy on troubleshooting would be greatly appreciated!

Thanks!
 
Last edited:
Apr 8, 2019
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Did you update your router firmware and what was your old modem?
Not sure why they recommended the SB6190.
I have not updated the router firmware since I purchased it. I am using DD-wrt, and didn't want to mess with it because the setup was somewhat complex with VNP, and other port management going on. I can't recall everything I set so I am somewhat afraid to tear it all down and start over.

My old modem was a SB6121. They said I needed at least 16x4 but said the 32x8 would be much better for 300MB service. May not be true since my old 4x4 seemed to have twice the actual throughput for some reason as the new 32x8. I have no idea if the modem plays any role in all this, but it is odd that it would actually slow the router down to change to a faster modem?
 
Try to load the factory firmware into the router.

Modern routers use a NAT assist feature to get the high speeds. I do not know if dd-wrt supports this, it might on certain chipset but I do not follow this closely.

I also do not know what speeds you should expect doing the NAT with the cpu since it is greatly impacted by the cpu clock rates and stuff. I know even the top routers will bottleneck under 300mbps without the NAT assist feature. Many low end routers will bottleneck at under 100mbps without the nat assist. This is how a inexpensive router can pass 1gbit of traffic wan-lan even with fairly low end cpu.

Most people do not even know about this until they get their first fast internet connection. Even on factory firmware you have to be careful what features you use, things like QoS or firewall filters force the traffic back through the cpu again.

This may not be your issue but 300mbps is going to be hard on a router without the assist active.
 
Apr 8, 2019
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Try to load the factory firmware into the router.

Modern routers use a NAT assist feature to get the high speeds. I do not know if dd-wrt supports this, it might on certain chipset but I do not follow this closely.

I also do not know what speeds you should expect doing the NAT with the cpu since it is greatly impacted by the cpu clock rates and stuff. I know even the top routers will bottleneck under 300mbps without the NAT assist feature. Many low end routers will bottleneck at under 100mbps without the nat assist. This is how a inexpensive router can pass 1gbit of traffic wan-lan even with fairly low end cpu.

Most people do not even know about this until they get their first fast internet connection. Even on factory firmware you have to be careful what features you use, things like QoS or firewall filters force the traffic back through the cpu again.

This may not be your issue but 300mbps is going to be hard on a router without the assist active.
Thanks for the input. I had no idea. I may end up re-flashing with the factory firmware, but I vaguely recall that I went with dd-wrt because it could handle our dynamic IP with DYNDNS and our VPN. It is going to be a pain to reconfigure all that as well as the VOIP, but I guess worth it if we get 6x internet speed at the end. I read a review of the router that said it was testing at 586MB WAN>LAN so it should be capable of it!
 
Be aware you are only going to get about 20mbps with vpn again for a similar reason. The cpu will bottleneck the traffic but in this case there is no hardware assist. Asus routers like 86u-2900 is one of the few that runs a newer cpu chip that has aes-ni support. Routers like these can get 300mbps vpn rates. These routers also support the merlin firmware which does not have all the features of dd-wrt but is much more stable.
 
Apr 8, 2019
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That is something to keep in mind. For now we only get 30mbps upstream anyhow, so it won't matter too much, but when we finally get truly fast upstream speed it will be time to upgrade!

Thanks for all your help! Any theory as to why I went from 120mbps before upgrading the modem to 50mbps after? That part still puzzles me!
 

mihen

Prominent
Oct 11, 2017
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Oh, don't reflash the firmware on your router. I just looked it up and there isn't any new firmware for it anyway. But resetting it and setting everything else back up while a pain could help.
The reason I am not so sure why they recommended the SB6190 is because it's a DOCSIS 3.0 modem. Cox supports DOCSIS 3.1 capable of gig speeds. Hopefully they were just trying to get rid of inventory and didn't charge you retail for it.
As for why you are at 50 mbps through your router with a new modem. From your troubleshooting already the thing that occurred to me may be custom settings on the device that aren't playing well connecting to a new modem like limiting ports. A cable modem bonds to a number of channels each capable of a certain speed. It may be how the traffic of a certain type is routed by the modem.
 
Apr 8, 2019
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To be clear they recommended a DOCSIS 3.1 modem, but said that for 300mbps I would be fine with a DOCSIS 3.1 16x4 but that a 24x8 would be better. The cheapest DOCSIS 3.1 I could find was $169 while I could get the 3.0 32x8 for $49 next day. Reports from this forum and others said that people were getting 500mbps with it so it seemed fine for my 300 plan.

I was going to reflash my modem today but was reading a lot of reports of bricked routers going from DD-WRT back to stock, and I am still not clear on the best way to do this. I got as far as backing up my DD-WRT configuration and doing screen dumps of all the setting pages.

Are you suggesting that I simply reset my DD-WRT back to default setting and see if that works, then rebuild the settings and see what (if anything) breaks it?

This seem like a much saner path, because I think I can just reload my present saved configuration and be no worse than I am today, with 50mbps, which is certainly functional.
 
Apr 8, 2019
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Ok, I reset to DD-WRT default settings, and there was no speed improvement. Still 50mbps. The good news is that I was able to restore settings from backup so no harm done.

Next, on a whim, I connected an old D-link DIR-865L with factory firmware and it gave speeds for about 500mbps on fast.com and about 320mbps on speedtest.net.

Maybe there is truth to the factory firmware/NAT assist theory? Or maybe I just have a bad Linksys router?
 

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