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Local network is causing 15Mbps internet speed drop.

Discussion in 'Networking' started by Alex Ethridge, Jul 1, 2019.

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  1. Alex Ethridge

    Alex Ethridge Thread Starter

    Apr 10, 2000
    When I connect the cable modem directly to one of my computers and check the speed, it is consistently about 15Mbps faster than when I go through my router. I have replaced the router and switch and this behavior persists even when everything on the network is turned off except the router and the switch and my desktop computer.

    How the network is laid out: The Spectrum cable modem has only one ethernet port. That port is connected by CAT5 cable to the eight-port switch. The eight-port switch is connected to my TV and router and the router is connected to my desktop computer, all by CAT5 cable. There is also a wireless printer.

    Is this normal?

    Why is this happening?
  2. Fireflycph


    Apr 1, 2016
    First Name:
    No, it's not normal to have such a drop. A slight drop is to be expected but never over 0,5Mps.
    I think you should check your router to see what the MTU is set for. The standard for Spectrum is, I believe, 1500, but I have had clients who had to lower it to 1492 and even one, just one, that had to lower it ever further.
    If tinkering with that doesn't change anything then try to set the Speed/Duplex for the network connection to something other than "Auto Detect"
    Without knowing the exact type of router and nic in the machine, not to mention which machine and OS then I'd start with 1Gb Full Duplex. Go from there to 1GB half duplex, then 100MB Full etc etc.
    Also try to change the ethernet cable out. Are you connected directly to the router or do you have other connectors in between? Such as a wall plate or such thing. THey can, doesn't happen often, go bad. Also the components used in those wiring schemes can be sub 5e. You'll still get a connection but not a good one. Here changing speed and duplex can help you see if there's an issue.

    Have you tried to connect straight to the router without using a switch or is this switch build-in to the router?

    Anyway, this is what I'd do, in order.

    • CHange the cable and connect straight to the router.
    • Fiddle with the MTU (Remember the original setting)
    • Experiment with the speed/duplex of the Nic. Use a new cable just to be sure.
    • Lastly, and this may be important. Is your modem set up in Bridge mode? Meaning that all traffic is just sent thru to the router. WHich then handles Firewall, DHCP MTU etc. etc.
    It'd be helpful if you could give us a layout of the cabling and the equipment attached to the network. Plus a list of the exact devices you have. From modem to Router to switch and then what you have attached. Plus the location of other devices on the network. I probably also should have asked if you experience the same drop when using other devices. Such as phones, tablets and the list goes on.

    I apologize for the seemingly endless blabbering and if I repeated/contradicted myself anywhere. It's getting late here and it's time for bed. :sleep:
  3. Couriant

    Couriant Trusted Advisor

    Mar 26, 2002
    First Name:
    Take out the switch, and turn of the wireless on your router and see what you get.

    What is your data plan and speed when direct connected?
  4. TerryNet

    TerryNet Moderator

    Mar 23, 2005
    First Name:
    Is the "Spectrum cable modem" actually a modem/router combo? If so, are it and the other router using different LAN subnets?

    Have you thought about eliminating the switch and any of the cables as culprits?
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