Speed drop if going through a router, fine if wired direct from modem

ziffel · 21 Apr 2019

I have a Docsis 3.0 motorola modem (provided by Spectrum). If I connect the ethernet from the modem direct to my PC, i immediately get full bandwidth, 120 Mbps, on all speedtests (fast.com etc). However, if I first go to my router, then ethernet to the PC, the speed drops to around 55 or 60 Mb. An important note here is that this combo has worked fine for several years. Same modem, same ethernet cable, same router. (I say that because it illustrates the older router, cable, and modem are capable of full my full bandwidth) This issue just popped up yesterday (Apr 20)

The oddity here is that purchasing and trying a brand new router with all the latest tech renders the exact same results. 55 to 60 Mb speed. The tests are very very consistent. Always the same each time, no variation. Same goes for wiring direct from modem to PC. Always 120 Mb.

The upload speed is always 100% good. Both direct to PC and through both routers, 12 Mb up.

The 55-60 speed (wired) is consistent on 3 different PC's in the house. Two are Win10, one is an older Win7 laptop. And I got that speed on the other PC's with the new router as well.

Here's what I've tried so far:

rebooting modem & router & PC (several times each)
different ethernet cable from modem to router and to PC (same results)
2nd, brand new router (same results)
disconnected all wires from modem (coax, ethernet, power) and re-connected
read through this thread. My symptom is similar, (120 --> 60 drop, download only), however I don't think it's relevant, because I get full speed as I should if I go direct from modem to PC ethernet in. If it were a software issue, I would think that would persist direct wired or via a router. Either way, none of the fixes in that thread helped me.
updated older router's firmware to latest, though even that update was from 4 years ago.

Thanks for any and all suggestions!

ram1220 · 21 Apr 2019

Your Spectrum modem might have recently received a software update that isn't playing nice with your router and network. I have Uverse and although I have not experienced the issue others have. The fix for Uverse was to change the address of the router to something other than what is automatically assigned to it. In other words the address of your Spectrum modem might be 192.168.1.2 (Just an example) The assigned address of the router might be along the lines of 192.168.1.xx. If you manually assign something other along the lines of 192.168.10.xx (Notice third number) it should solve your download slowness. You will need to reboot all of your connected devices after rebooting your router as their DHCP addresses will change. I'm not a network guy so I hope I explained this properly. With Uverse this was causing a double NAT issue that was slowing downloads. Spectrum might have pushed a similar update to their modems. It's something to try. Others might have better things to try.

ziffel · 22 Apr 2019

Thanks, @ram1220. I'm not where I can try your suggestion at the moment, but look forward to trying it this evening. I'll report back.

- - - Updated - - -
@ram1220 thanks for the suggestion, but unfortunately, it had no effect. Just to be clear, I changed the router's IP from 192.168.0.1 to 192.168.10.1, powered off the modem, the router, and my pc, then powered the modem on, then router, then PC. everything is working, but i still get 60 Mb out of 120.

- - - Updated - - -

I "solved" this by just having the cable co. swap out the modem for a new one.

ram1220 · 22 Apr 2019

I'm glad you were able to solve the issue.