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Thread: connecting gigabit switch to modem = slower download speed tests

  1. #1
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    connecting gigabit switch to modem = slower download speed tests

    i experienced a download speed problem when i tried using a gigabit switch instead of a 10 / 100 switch to connect my cable modem

    i just got a motorolla sb6120 cable modem and it has a gigabit capable port -

    if i connect the computer gigabit nic directly to the cable modem i get good speed tests 34 - 37 mbps down and 6 - 8 mbps up .


    i use two switches and a 10 / 100 router for the usual connections to my lan and modem

    for years i have used this configuration

    the first switch ( an at least 13 year old 10 / 100 linksys switch ( the first one i ever bought ) )

    this switch has the modem the router and a ( 10 / 100 ) server nic connected - this way the router gets an ip address and one of the server nics get an ip address

    the second switch is a gigabit switch and it is connected to all the comptuers and the router

    in this configuration - i also get good speed tests from all the computers

    now the funny part comes in when i try to replace the 10 / 100 switch with a gigabit switch -

    for some reason when i do a speed test - i only get about 13 - 22 mbps down - the 6 - 8 mbps up stays good

    i have tried this with two different gigabit switches and both give the same results -

    i don't understand why the gigabit switch would cause that - i tried taking out the server nic

    i even tried connecting the router to the 10 / 100 switch and then connect that switch to the first gigabit switch that is connected to the modem

    same speed issue - why would just the introduction of a gigabit switch connection to the modem cause a dramatic speed test reduction ?

    oh yeah - the main reason i want a gig connect to the modem is because the sb120 shows a red activity light for 10 / 100 and a cool blue light for the gig connect - the red light clashes with the other green and blue lights on the modem - lol
    Last edited by edlogic; 04-29-09 at 05:27 PM.

  2. #2
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    I recently went through this with my ISP at work as well. We just installed a TimeWarner DAF node here at work which uses fiber instead of coax to deliver the signal to the modem. Once the on-site techs were done installing the modem we began to perform our speed tests to check the line and noticed that the speeds were horrible. 1.5 - 3 Mbps. We had shown this to the on-site installers so they hooked their laptop up to the modem directly and we noticed the speeds were through the roof! (50Mbps+) So we ended up thinking it was our routing equipment. Well we ended up changing all of our equipment. (it was all old stuff anyway) But once we got everything going we noticed the speeds were still the same. We weren't getting anywhere near the speeds we were paying for. Timewarner came back on site and hooked their laptop up to the line again and, sure enough, it was going full blast. After picking the technicians brain for a few minutes we found out that there is a "known" issue with their modems and their ability to properly negotiate link speeds with other devices. The technician actually had forced the link speed on his laptops NIC to 100/full which allowed him to get these incredible speeds. We had them change the modem/DAF node to something else thinking that the problem was brand specific but that didn't help either. The solution for this was the FORCE the NIC speed of the WAN port to 100/Full duplex on the router and leave all the other ports on the router to auto negotiate. Once we did this the speeds went through the roof and we were actually getting faster speeds than what we were paying for. It's unfortunate but you'll probably need to change your router to something with more control so you can force speeds on individual ports. Most home user routers won't have these features. My D-Link DIR-655 router at home lets you set the speed on the WAN port to 10/100/1000 or auto negotiate but doesn't give you the half/full option. You'll need the ability to force 100/full. It took us tons of trial & error along with replacing equipment to get the right balance we have now. I'll keep monitoring this post for a while to see if you have any further questions about what we did. I can also send you a Visio diagram of our setup if you think it will help. But I warn you it's fairly complex.

  3. #3
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    thanks for the reply -
    i don't have any questions for you - you were very informative
    it gives me the notion that maybe the main resource i should tap is motorolla and ask them what gives with their modem and gigabit interfacing

  4. #4
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    Another thing you could try is to log in to the modem itself and see if there are any options to set the NIC speed to 100/Full to match the speed of the WAN port on your router. Just a thought. Good luck.

  5. #5
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    no options for that - the only two things the modem lets you do is reset to factory defaults and reboot -

    i have contacted motorolla and they seem very responsive - waiting for secondary reply now -

    http://broadband.motorola.com/consumers/support/

  6. #6
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    connecting gigabit switch to modem = slower download speed tests

    hi,

    its very useful to me....thank u.........

  7. #7
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    connecting gigabit switch to modem = slower download speed tests

    hi,

    its very useful to me....thank u.........


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  8. #8
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    oh well - motorolla just said - try changing things around - no help

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