Equipment (Modems,Gateways)
Modems, Gateways, and Networking Devices
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Comcast modem won't talk to new gig switch

We have a Comcast modem / router (Cisco DPC3941B, 4 port, DOCSIS 3.0) and on the LAN side, a 24 port 10/100 netgear switch. Everything works great. We went to upgrade the swich to a Ntegear 100/1000 switch. I plugged everything in and the LAN side seems to connect fine, but the Comcast modem doesn't seem to want to talk to the new switch (no internet connection). I can plug a computer directly into one of the modem ports and get a gig connection, but can't get an internet connection through the switch. The switch is un-managed, so should just be plug and play. Am I missing something?  I put the old switch back in for now and everything works fine. 

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Re: Comcast modem won't talk to new gig switch

Before replacing the switch, have you tried a different ethernet cable from the modem to the new switch? I have seen some defective ethernet cables that have slight damage to 1 or 2 of the individual strands, such that they could operate at 100mbps but not at gigabit.

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Re: Comcast modem won't talk to new gig switch

Yep, they absolutely do. The crux of it is that 100mbps operation uses only 4 wires inside the ethernet cable (specifically wires #1,2,3, and 6). Gigabit uses all 8 wires.

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Re: Comcast modem won't talk to new gig switch

Hello Patty and welcome,

 

As long as your new 100/1000 Netgear switch is unmanaged and does not have WAN interface that requires configuration, it should work fine. You must make sure that the switch i/o interface interconnect is plugged into any of the DPC3941 Lan Ports 1-4. I would try to reset the DPC first, then POR the switch, then reboot all computers directly connected to the switch and you should be up and running. Sometimes, if you do not initialize everything using this sequence that is stale network interconnect info that could prevent your computers from getting a valid dynamic address from the DPC3941.

 

Hope this helps you out.

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Re: Comcast modem won't talk to new gig switch

Thanks for your reply. I rebooted everything as you suggested and still can't get an internet connection through the switch. The port on the switch were the modem is plugged in does eventually go solid green, indicating a gig connection, but I can't log onto the modem or get any response from a browser. I had only one computer, the switch and modem/router plugged in to make sure there was no other interferance going on. Then I plugged another computer into the switch and could connect and transfer files from one computer to the other through the switch.  I plugged a computer directly into the modem and get a gig internet connection. I put the old 10/100 switch back in and it works fine. I'm ready to send this switch back. I opened a support request with Netgear to see if they have any suggestions. Thanks agian for your help. If you can think of anything else I can try, please let me know.

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Re: Comcast modem won't talk to new gig switch

As long as you have the correct 100/1000 switch hub port interface connected to any of the DPC3941 Lan Ports and you know it is definitely un-managed, then it sounds like a defective switch.

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Re: Comcast modem won't talk to new gig switch

Before replacing the switch, have you tried a different ethernet cable from the modem to the new switch? I have seen some defective ethernet cables that have slight damage to 1 or 2 of the individual strands, such that they could operate at 100mbps but not at gigabit.

View solution in original post

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Re: Comcast modem won't talk to new gig switch

Yes! I talked to Netgear and they suggested putting a new CAT6 cable from the modem to the switch. Works perfectly now. Woohoo!  Apparently cables do mater.

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Re: Comcast modem won't talk to new gig switch

Yep, they absolutely do. The crux of it is that 100mbps operation uses only 4 wires inside the ethernet cable (specifically wires #1,2,3, and 6). Gigabit uses all 8 wires.

View solution in original post

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